After a discussion about language specs, this seems like the best way to
go, because it's simpler to reason about both for humans and compilers.

The `bitcast_result_ptr` ZIR instruction is no longer needed.

This commit also implements writing enums, arrays, and vectors to
virtual memory at compile-time.

This unlocked some more of compiler-rt being able to build, which
in turn unlocks saturating arithmetic behavior tests.

There was also a memory leak in the comptime closure system which is now
fixed.