Tweak: switch to short type names in op names

[lukewagner]

One significant remaining divergence between ml-proto and design is that ml-proto uses short type names (i32 instead of int32). We've agreed that we want opcode names to exactly match AstSemantics.md, so one side needs to change. I actually like the short names better (shorter and no added ambiguity), so rather than changing ml-proto to match, how about we change design to use the short type names?

[sunfishcode]

sgtm

[jfbastien]

Agreed.

[lukewagner]

Making a new issue for this was probably overkill. I'll close this issue and we can discuss in #357.