compiler: simplify interface lowering

This commit simplifies the IR a little bit: instead of calling
pseudo-functions runtime.interfaceImplements and
runtime.interfaceMethod, real declared functions are being called that
are then defined in the interface lowering pass. This should simplify
the interaction between various transformation passes. It also reduces
the number of lines of code, which is generally a good thing.

src/runtime/interface.go

@@ -117,6 +117,10 @@ type typecodeID struct {
 	// Keeping the type struct alive here is important so that values from
 	// reflect.New (which uses reflect.PtrTo) can be used in type asserts etc.
 	ptrTo *typecodeID
+
+	// typeAssert is a ptrtoint of a declared interface assert function.
+	// It only exists to make the rtcalls pass easier.
+	typeAssert uintptr
 }
 
 // structField is used by the compiler to pass information to the interface
@@ -133,11 +137,3 @@ type structField struct {
 // asserts. Also, it is replaced with const false if this type assert can never
 // happen.
 func typeAssert(actualType uintptr, assertedType *uint8) bool
-
-// Pseudo function call that returns whether a given type implements all methods
-// of the given interface.
-func interfaceImplements(typecode uintptr, interfaceMethodSet **uint8) bool
-
-// Pseudo function that returns a function pointer to the method to call.
-// See the interface lowering pass for how this is lowered to a real call.
-func interfaceMethod(typecode uintptr, interfaceMethodSet **uint8, signature *uint8) uintptr