separate module decoding state/methods into their own struct

[marler8997]

This change moves the state (fixed buffer stream) and methods (decode/read) around decoding a module into a separate struct Decoder so they can be discarded after the module is decoded.

Along with this change, I also fixed a few more things I noticed along the way:

• removed version field from Module (unused)
• combined common code between min/max limit cases
• rename module field to wasm_bin on Module struct to distinguish the wasm binary buffer from the higher level module object
• fixed readSlice so it doesn't call skipBytes which unnecessarily copies the entire slice and throws it away
• add .zig-cache to .gitignore for zig 0.13.0

[malcolmstill]

Thanks for this @marler8997