compiler: add support for new unsafe slice/string functions

This adds support for unsafe.SliceData, unsafe.String, and
unsafe.SringData that were introduced in Go 1.20.

compiler/testdata/go1.20.go

@@ -0,0 +1,15 @@
+package main
+
+import "unsafe"
+
+func unsafeSliceData(s []int) *int {
+	return unsafe.SliceData(s)
+}
+
+func unsafeString(ptr *byte, len int16) string {
+	return unsafe.String(ptr, len)
+}
+
+func unsafeStringData(s string) *byte {
+	return unsafe.StringData(s)
+}