revset: make tokenize extensible to parse alias declarations and definitions Before this patch, "tokenize" doesn't recognize the symbol starting with "$" as a valid one. This prevents revset alias declarations and definitions from being parsed with "tokenize", because "$" may be used as the initial letter of alias arguments. BTW, the alias argument name doesn't require leading "$" itself, in fact. But we have to assume that users may use "$" as the initial letter of argument names in their aliases, because examples in "hg help revsets" uses such names for a long time. To make "tokenize" extensible to parse alias declarations and definitions, this patch introduces optional arguments "syminitletters" and "symletters". Giving these sets can change the policy of "valid symbol" in tokenization easily. This patch keeps original examination of letter validity for reviewability, even though there is redundant interchanging between "chr"/"ord" at initialization of "_syminitletters" and "_symletters". At most 256 times examination (per initialization) is cheaper enough than revset evaluation itself. This patch is a part of preparation for parsing alias declarations and definitions more strictly.