Idea: aggressive comptime specialisation

[malcolmstill]

• Different users may have different requirements / constraints
• Allow the consumer of the library to configure, at compile time, certain limits. E.g. if they know they will only be running small .wasm binaries, maybe they want the array of instructions to only of length less than can be represented by a u16 and thereby cut down on the size of the instructions themselves
• More examples:
  • Customise how memory works:
    • mmap with guard pages