wip-rtl: macro-instructions, constants loading

[Andy Wingo]

Another brief note before I go off to do other things.

I reworked the interface of the assembler to allow for
macro-instructions: instructions that don't correspond directly to a
particular opcode.  There are new macro-instructions for the function
prologue, loading constants of any kind, initializing slots of
constants, and emitting labels.  The basic instructions like
make-non-immediate now take labels again instead of constants.

With this, I was able to wire up the init procedure to relocate the
constants.  I've added a test case, and things are working for most
kinds of values.  Notable exceptions are arrays, uniform and otherwise.

OK, off to other things!

Andy
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