YJIT: shrink Context down to 15 bytes

[maximecb]

Fairly straightforward refactoring. We could probably turn TempMapping back into an enum, but it seemed simpler to just keep it as is for now.

master (17 byte context):
  yjit_alloc_size:          18,817,801
with patch (15 byte context):
  yjit_alloc_size:          18,440,141

2% memory savings (wow!)

Next year we may want to look at using a crate that has support for managing bit fields, or a more sophisticated way to encode the context if we want to squeeze the maximum out of those precious bits 🤔