interp: rewrite entire package

For a full explanation, see interp/README.md. In short, this rewrite is
a redesign of the partial evaluator which improves it over the previous
partial evaluator. The main functional difference is that when
interpreting a function, the interpretation can be rolled back when an
unsupported instruction is encountered (for example, an actual unknown
instruction or a branch on a value that's only known at runtime). This
also means that it is no longer necessary to scan functions to see
whether they can be interpreted: instead, this package now just tries to
interpret it and reverts when it can't go further.

This new design has several benefits:

  * Most errors coming from the interp package are avoided, as it can
    simply skip the code it can't handle. This has long been an issue.
  * The memory model has been improved, which means some packages now
    pass all tests that previously didn't pass them.
  * Because of a better design, it is in fact a bit faster than the
    previous version.

This means the following packages now pass tests with `tinygo test`:

  * hash/adler32: previously it would hang in an infinite loop
  * math/cmplx: previously it resulted in errors

This also means that the math/big package can be imported. It would
previously fail with a "interp: branch on a non-constant" error.

interp/interp_test.go

@@ -42,9 +42,29 @@ func runTest(t *testing.T, pathPrefix string) {
 	// Perform the transform.
 	err = Run(mod, false)
 	if err != nil {
+		if err, match := err.(*Error); match {
+			println(err.Error())
+			if !err.Inst.IsNil() {
+				err.Inst.Dump()
+				println()
+			}
+			if len(err.Traceback) > 0 {
+				println("\ntraceback:")
+				for _, line := range err.Traceback {
+					println(line.Pos.String() + ":")
+					line.Inst.Dump()
+					println()
+				}
+			}
+		}
 		t.Fatal(err)
 	}
 
+	// To be sure, verify that the module is still valid.
+	if llvm.VerifyModule(mod, llvm.PrintMessageAction) != nil {
+		t.FailNow()
+	}
+
 	// Run some cleanup passes to get easy-to-read outputs.
 	pm := llvm.NewPassManager()
 	defer pm.Dispose()