Add encodingToStrictByteString

[phadej]

This runs Builder to produce Strict ByteString directly, by making a mutable buffer and growing it exponentially.

This might be good or bad, better or worse than
LBS.toStrict . encodingToLazyByteString.
Latter allocates many small chunks, and copies once; encodingToStrictByteString makes a buffer exponentially, but copies data everytime.

TL;DR bytestrings Builder implementation allows to execute it into a contiguous buffer, i.e. strict bytestring.

(We could almost reuse the same trick to produce strict text as well, but we'd need to copy to unpin resulting bytearray)

[phadej]

Please don't merge this. This wasn't any more efficient than serializing to lazy bytestring and then converting to strict one if needed.