Tracking issue for bit arrays on javascript target

There are still many features of bit arrays that are not yet implemented on the javascript target. We want to have all of them eventually, but it's a lot to implement in one go, and a lot to have a separate issue for each one. So I've decided to create a tracking issue to list them all, and these can be slowly ticked off when people implement specific features.

• Non byte-aligned access
• Non-constant sizes when pattern matching: <<bits:size(some_variable)>>
• Non byte-aligned segments
• Non-utf8 strings: utf16, utf32, utf16_codepoint and utf32_codepoint
• Pattern matching on utf8_codepoint, utf16_codepoint and utf32_codepoint
• Signedness: signed and unsigned
• Endianness: Missing native
• Unit specification: unit
• 16-bit floats

Please let me know if I've missed any. I'm not super familiar with bit array syntax so I mostly just looked through the compiler and found where an error was thrown on javascript.