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/testdata/map.ll

@@ -48,8 +48,7 @@ entry:
 define internal void @main.testNonConstantBinarySet() {
   %hashmap.key = alloca i8
   %hashmap.value = alloca i8
-  ; Create hashmap from global. This breaks the normal hashmapBinarySet
-  ; optimization, to test the fallback.
+  ; Create hashmap from global.
   %map.new = call %runtime.hashmap* @runtime.hashmapMake(i8 1, i8 1, i32 1, i8* undef, i8* null)
   store %runtime.hashmap* %map.new, %runtime.hashmap** @main.binaryMap
   %map = load %runtime.hashmap*, %runtime.hashmap** @main.binaryMap
@@ -64,8 +63,7 @@ define internal void @main.testNonConstantBinarySet() {
 ; operations (with string keys).
 define internal void @main.testNonConstantStringSet() {
   %hashmap.value = alloca i8
-  ; Create hashmap from global. This breaks the normal hashmapStringSet
-  ; optimization, to test the fallback.
+  ; Create hashmap from global.
   %map.new = call %runtime.hashmap* @runtime.hashmapMake(i8 8, i8 1, i32 1, i8* undef, i8* null)
   store %runtime.hashmap* %map.new, %runtime.hashmap** @main.stringMap
   %map = load %runtime.hashmap*, %runtime.hashmap** @main.stringMap