This adds an optimization to EmitPointerPack when all values are constants.
The values are stored in a constant global instead of on the heap.
This eliminates a heap allocation, and can reduce binary size.
This commit makes a number of changes:
* It avoids a dependency on Compiler.emitStartGoroutine.
* It moves the func-lowering pass to the transform package.
* It adds testing to func lowering.
No functionality should have changed with this commit.
This code is required by transformation passes which are being moved
into a separate package, but is too complicated to simply copy.
Therefore, I decided to move them into a new package.
