compiler: refactor func lowering to the transform package

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.

transform/func-lowering_test.go

@@ -0,0 +1,10 @@
+package transform
+
+import (
+	"testing"
+)
+
+func TestFuncLowering(t *testing.T) {
+	t.Parallel()
+	testTransform(t, "testdata/func-lowering", LowerFuncValues)
+}