[G-API] Fixed Coverity issues

* Fixed Coverity issues
 - VectorRef&OpaqueRef m_kind = CV_UNKNOWN
 - added same-type overload for saturate()
 - sanitized resize value in ByteMemoryInStream::operator>> (std::string& str)
 - handled throws from ~GStreamingExecutor()

* Catching exception by const ref

* Addressing Sergey's comments

* Applied enable_if semanitcs to saturate(x, round) too

* Removed uncaught_exception, made destructor noexcept back

* Split Fluid ConvertTo to multiple functions to avoid ifs; added CV_ALWAYS_INLINE

* Added FIXME to address throwings from stop()

* Fix standalone

* Addressing comments

* Guarded SIMD optimizations properly

* Removed excess parameter from simd_impl functions