golang_str: tests: Deep replacer

deepReplace returns object's clone with replacing all internal objects
selected by predicate via provided replacement function. We will use
this functionality in the following patches to organize testing of
bstr/ustr methods: a method would be first invoked on regular str, and
then on bstr/ustr and the result will be compared against each other.
The results are usually different, because e.g. u'a b c'.split() returns
[u'a', u'b', u'c'] while b('a b c').split() should return
[b('a'), b('b'), b('c')]. We want to make sure that the second result is
exactly the first result with all instances of unicode replaced by bstr.
That's where deep replacer will be used.

The deep replacement itself is implemented via pickle reduce/rebuild
protocol: we unassemble and reconstruct objects. And while an object is
unassembled, we try to apply the replacement recursively. Since this is
not so trivial functionality, it itself also comes with a test.