revlog: avoid some unnecessary seek/tell syscalls

manifest: speed up creation of the manifestdict - fold iteration and rawset into parse - avoid creating extra new strings with [:] where possible - speed up node.bin

lazyparser.findnode: fix typo and s/rfind/find/ There's no reason to use reverse string search and it's slightly slower

lazyindex: speed up __getitem__ This function is fairly performance sensitive, so we make a couple ugly tweaks: - keep all entries packed so we needn't test entry types - fold index lookup/load into unpack call to eliminate local variable setting

lazyparser: up the blocksize from 512 bytes to 64k

revlog: implement a fast path for heads

revlogio: speed up parsing - precalcuate ending offset - pull some variables into local scope - separate inline and out of line code paths