This disentangles the control flow between Action View's `translate` and
I18n's `translate`.  In doing so, it fixes a handful of corner cases,
for which tests have now been added.  It also reduces memory
allocations, and improves speed when using a default:

**Memory**

```ruby
require "benchmark/memory"

Benchmark.memory do |x|
  x.report("warmup") { translate(:"translations.foo"); translate(:"translations.html") }
  x.report("text") { translate(:"translations.foo") }
  x.report("html") { translate(:"translations.html") }
  x.report("text 1 default") { translate(:"translations.missing", default: :"translations.foo") }
  x.report("html 1 default") { translate(:"translations.missing", default: :"translations.html") }
  x.report("text 2 defaults") { translate(:"translations.missing", default: [:"translations.missing", :"translations.foo"]) }
  x.report("html 2 defaults") { translate(:"translations.missing", default: [:"translations.missing", :"translations.html"]) }
end
```

Before:

```
                text     1.240k memsize (     0.000  retained)
                        13.000  objects (     0.000  retained)
                         2.000  strings (     0.000  retained)
                html     1.600k memsize (     0.000  retained)
                        19.000  objects (     0.000  retained)
                         2.000  strings (     0.000  retained)
      text 1 default     4.728k memsize (     1.200k retained)
                        39.000  objects (     4.000  retained)
                         5.000  strings (     0.000  retained)
      html 1 default     5.056k memsize (     1.160k retained)
                        41.000  objects (     3.000  retained)
                         4.000  strings (     0.000  retained)
     text 2 defaults     7.464k memsize (     2.392k retained)
                        54.000  objects (     6.000  retained)
                         4.000  strings (     0.000  retained)
     html 2 defaults     7.944k memsize (     2.384k retained)
                        60.000  objects (     6.000  retained)
                         4.000  strings (     0.000  retained)
```

After:

```
                text   952.000  memsize (     0.000  retained)
                         9.000  objects (     0.000  retained)
                         1.000  strings (     0.000  retained)
                html     1.008k memsize (     0.000  retained)
                        10.000  objects (     0.000  retained)
                         1.000  strings (     0.000  retained)
      text 1 default     2.400k memsize (    40.000  retained)
                        24.000  objects (     1.000  retained)
                         4.000  strings (     0.000  retained)
      html 1 default     2.464k memsize (     0.000  retained)
                        22.000  objects (     0.000  retained)
                         2.000  strings (     0.000  retained)
     text 2 defaults     3.232k memsize (     0.000  retained)
                        30.000  objects (     0.000  retained)
                         2.000  strings (     0.000  retained)
     html 2 defaults     3.456k memsize (     0.000  retained)
                        32.000  objects (     0.000  retained)
                         2.000  strings (     0.000  retained)
```

**Speed**

```ruby
require "benchmark/ips"

Benchmark.ips do |x|
  x.report("text") { translate(:"translations.foo") }
  x.report("html") { translate(:"translations.html") }
  x.report("text 1 default") { translate(:"translations.missing", default: :"translations.foo") }
  x.report("html 1 default") { translate(:"translations.missing", default: :"translations.html") }
  x.report("text 2 defaults") { translate(:"translations.missing", default: [:"translations.missing", :"translations.foo"]) }
  x.report("html 2 defaults") { translate(:"translations.missing", default: [:"translations.missing", :"translations.html"]) }
end
```

Before:

```
                text     35.685k (± 0.7%) i/s -    179.050k in   5.017773s
                html     28.569k (± 3.1%) i/s -    143.871k in   5.040128s
      text 1 default     13.953k (± 2.0%) i/s -     70.737k in   5.071651s
      html 1 default     12.507k (± 0.4%) i/s -     63.546k in   5.080908s
     text 2 defaults      9.103k (± 0.3%) i/s -     46.308k in   5.087323s
     html 2 defaults      8.570k (± 4.3%) i/s -     43.071k in   5.034322s
```

After:

```
                text     36.694k (± 2.0%) i/s -    186.864k in   5.094367s
                html     30.415k (± 0.5%) i/s -    152.900k in   5.027226s
      text 1 default     18.095k (± 2.7%) i/s -     91.086k in   5.036857s
      html 1 default     15.934k (± 1.7%) i/s -     80.223k in   5.036085s
     text 2 defaults     12.179k (± 0.6%) i/s -     61.659k in   5.062910s
     html 2 defaults     11.193k (± 2.1%) i/s -     56.406k in   5.041433s
```