[Implement] Buffer.compare

[jtenner]

Notes:

• this function works really nicely with memory.compare()
• Currently, the compiler doesn't like using static functions as function parameters (Will open an issue in the AssemblyScript repo)

ERROR AS218: Cannot access method 'compare' without calling it as it requires 'this' to be set.

     actual.sort(Buffer.compare);
                 ~~~~~~~~~~~~~~
 in tests/buffer.spec.ts(60,16)

Edit:

• Switching to static readonly function expression causes this to compile

[jtenner]

With the static function fix, everything seems to look nice. Thoughts? @dcodeIO

[jtenner]

TODO: Implement Buffer#compare.
TODO: Return 1, 0, or -1