floating point support

[graydon]

Currently there is zero support for floating point past ... perhaps the lexer
and a few of the machine-types? The IL translator, register allocator and
instruction selectors, at very least, have no clue. Threading the type through
the compiler shouldn't be too brutal -- a little tedious -- but might not be
fruitful to go all the way on this backend, as it may not live long enough to
pay for itself (as with u64 and s64). Not sure.

[graydon]

Shifted to rustc. It's still relevant there.

[graydon]

tjc and lkuper implemented this in a sequence of commits ending around a47cd50