compiler: fix difference in aliases in interface methods

There used to be a difference between `byte` and `uint8` in interface
methods. These are aliases, so they should be treated the same.
This patch introduces a custom serialization format for types,
circumventing the `Type.String()` method that is slightly wrong for our
purposes.

This also fixes an issue with the `any` keyword in Go 1.18, which
suffers from the same problem (but this time actually leads to a crash).

testdata/interface.txt

@@ -20,6 +20,7 @@ Stringer.String(): foo
 Stringer.(*Thing).String(): foo
 s has String() method: foo
 nested switch: true
+Byte():  3
 non-blocking call on sometimes-blocking interface
 slept 1ms
 slept 1ms