builder: run interp per package

This results in a significant speedup in some cases. For example, this
runs over twice as fast with a warm cache:

    tinygo build -o test.elf ./testdata/stdlib.go

This should help a lot with edit-compile-test cycles, that typically
only modify a single package.

This required some changes to the interp package to deal with globals
created in a previous run of the interp package and to deal with
external globals (that can't be loaded from or stored to).

interp/errors.go

@@ -20,6 +20,11 @@ var (
 	errMapAlreadyCreated      = errors.New("interp: map already created")
 )
 
+// This is one of the errors that can be returned from toLLVMValue when the
+// passed type does not fit the data to serialize. It is recoverable by
+// serializing without a type (using rawValue.rawLLVMValue).
+var errInvalidPtrToIntSize = errors.New("interp: ptrtoint integer size does not equal pointer size")
+
 func isRecoverableError(err error) bool {
 	return err == errIntegerAsPointer || err == errUnsupportedInst || err == errUnsupportedRuntimeInst || err == errMapAlreadyCreated
 }