stringutil: add function to pretty print an object This is inspired by the pprint() module/function (which we can't use because the output is different on Python 2 and 3 - namely the use of b'' literals). We hook it up to `hg debugwireproto` for printing the response to a wire protocol command. This foreshadows future peer work, which will support decoding CBOR responses into rich data structures. Differential Revision: https://phab.mercurial-scm.org/D2987