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.

Makefile

@@ -177,9 +177,11 @@ tinygo-test:
 	$(TINYGO) test encoding/ascii85
 	$(TINYGO) test encoding/base32
 	$(TINYGO) test encoding/hex
+	$(TINYGO) test hash/adler32
 	$(TINYGO) test hash/fnv
 	$(TINYGO) test hash/crc64
 	$(TINYGO) test math
+	$(TINYGO) test math/cmplx
 	$(TINYGO) test text/scanner
 	$(TINYGO) test unicode/utf8