Previously, EmitPointerPack would generate an out-of-bounds read from an
alloca. This commit fixes that by creating an alloca of the appropriate
size instead of using the size of the to-be-packed data (which might be
smaller than a pointer).
I discovered this error while working on a rewrite of the interp
package, which checks for out-of-bounds reads and writes. There I
discovered this issue when the image package was compiled.
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.
