fix #6540: IOBuffer(s) only for ByteString, utf16(s) for binary data, and add is_valid_utf16(s)
check for BOM in UTF-16 conversion of byte arrays
For big types, only use unpromoted arithmetic for types no larger than long for that system. Use explicit integer sizes in random test to avoid errors on 32 bit long systems.
Add documentation about custom exceptions to the manual
Merge pull request #6546 from stevengj/utf16_data

fix #6540: utf16(s) for binary data, etc.
Merge pull request #6595 from nalimilan/exceptionsdocs

Add documentation about custom exceptions to the manual
Merge pull request #6583 from JuliaLang/anj/big32

Only use unpromoted arithmetic for types no larger than 32 bit on 32 bit systems. Use explicit integer sizes in random test to avoid errors on 32 bit systems.
Fix int32 chokepoint in jl_uv.c for #6567