Given the change in 0c20483853, this is no longer necessary, since
we know that one instance will always be running if we're executing a
given block of code.
Fixes issue #73. Previously, if we had e.g. an int16 type that was being shifted
left by 1, then the constant integer 1 would come in as an int32, we'd convert
the int16 to an int32, and then we'd do the shift. Now, for shifts, the type
of the expression is always the same as the type of the value being shifted.
This commit adds support for swizzles like "foo.zy" (if "foo" is,
for example, a float<3> type) as rvalues. (Still need support for
swizzles as lvalues.)
This way, we match C/C++ in that casting a bool to an int gives either the value
zero or the value one. There is a new stdlib function int sign_extend(bool)
that does sign extension for cases where that's desired.
