main: add -print-allocs flag that lets you print all heap allocations

This flag, if set, is a regexp for function names. If there are heap
allocations in the matching function names, these heap allocations will
be printed with an explanation why the heap allocation exists (and why
the object can't be stack allocated).

transform/testdata/allocs2.go

@@ -0,0 +1,47 @@
+package main
+
+func main() {
+	n1 := 5
+	derefInt(&n1)
+
+	// This should eventually be modified to not escape.
+	n2 := 6 // OUT: object allocated on the heap: escapes at line 9
+	returnIntPtr(&n2)
+
+	s1 := make([]int, 3)
+	readIntSlice(s1)
+
+	s2 := [3]int{}
+	readIntSlice(s2[:])
+
+	// This should also be modified to not escape.
+	s3 := make([]int, 3) // OUT: object allocated on the heap: escapes at line 19
+	returnIntSlice(s3)
+
+	_ = make([]int, getUnknownNumber()) // OUT: object allocated on the heap: size is not constant
+
+	s4 := make([]byte, 300) // OUT: object allocated on the heap: object size 300 exceeds maximum stack allocation size 256
+	readByteSlice(s4)
+}
+
+func derefInt(x *int) int {
+	return *x
+}
+
+func returnIntPtr(x *int) *int {
+	return x
+}
+
+func readIntSlice(s []int) int {
+	return s[1]
+}
+
+func readByteSlice(s []byte) byte {
+	return s[1]
+}
+
+func returnIntSlice(s []int) []int {
+	return s
+}
+
+func getUnknownNumber() int