deprecate Bytes::blit_from_string and Bytes::set_utf16_char

[Yu-zh]

This PR:

1. deprecated Bytes::blit_from_string and Bytes::set_utf16_char
2. refactored stringbuilder_buffer.mbt to use FixedArray[Byte] instead of Bytes
3. Added FixedArray::reinterpreted_as_bytes and Bytes::reinterpreted_as_fixedarray for non-copy conversion between the two types.

[coveralls]

Pull Request Test Coverage Report for Build 4062

Details

• 9 of 11 (81.82%) changed or added relevant lines in 5 files are covered.
• 7 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.1%) to 80.287%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
buffer/buffer.mbt	2	4	50.0%

Files with Coverage Reduction	New Missed Lines	%
bytes/bytes.mbt	1	77.27%
builtin/bytes.mbt	6	52.63%

Totals
Change from base Build 4047:	-0.1%
Covered Lines:	4370
Relevant Lines:	5443

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💛 - Coveralls