Use smaller variable names for frequently-used variables

[kripken]

The emscripten minifier could do a better job if it made frequently-used variables use the first names, which tend to be shorter (a,b,c etc, instead of two-letter names or worse).

We could do this with a new simple pass that runs near the end of the pipeline (after minify), and just replaces local names in a function based on usage, most frequently-appearing variables get the first names, etc.

[stale]

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