all: refactor heap-to-stack transform into the transform package

Also add unit tests.

This is the first of several transformation (optimization/lowering)
passes that I'd like to move to the new transform package. This
separates the compiler from the optimizer.

Also, it finally adds unit tests for the compiler, not just end-to-end
compilation tests. This should improve robustness and should make it
easier to change these transformation passes in the future.
While the heap-to-stack transform is relatively simple, other passes are
much more complex. Adding unit tests not only helps robustness over
time, but also doubles as documentation as to what these transformation
passes do exactly.

transform/transform.go

@@ -0,0 +1,13 @@
+// Package transform contains transformation passes for the TinyGo compiler.
+// These transformation passes may be optimization passes or lowering passes.
+//
+// Optimization passes transform the IR in such a way that they increase the
+// performance of the generated code and/or help the LLVM optimizer better do
+// its job by simplifying the IR. This usually means that certain
+// TinyGo-specific runtime calls are removed or replaced with something simpler
+// if that is a valid operation.
+//
+// Lowering passes are usually required to run. One example is the interface
+// lowering pass, which replaces stub runtime calls to get an interface method
+// with the method implementation (either a direct call or a thunk).
+package transform