No need to shadow Topcommon.evaluation_result

The phrase_name ref is updated with a new unique name at the start of
Topeval.execute_phrase. All the remaining functions called then use this
new value, but beforehand retrieved it from the ref. Making it an
argument to Topeval.load_lambda and passing it on to Topeval.load makes
the data flow slightly more explicit and also eliminates the need for an
accessor function for phrase_name ref later.

This commit simply moves code from Topeval (whose interface is shared
with the bytecode interpreter) to a new module Tophooks which is
specific to the native toplevel.