parsers: use base-16 trie for faster node->rev mapping This greatly speeds up node->rev lookups, with results that are often user-perceptible: for instance, "hg --time log" of the node associated with rev 1000 on a linux-2.6 repo improves from 0.3 seconds to 0.03. I have not found any instances of slowdowns. The new perfnodelookup command in contrib/perf.py demonstrates the speedup more dramatically, since it performs no I/O. For a single lookup, the new code is about 40x faster. These changes also prepare the ground for the possibility of further improving the performance of prefix-based node lookups.