revset: introduce "_parsealiasdecl" to parse alias declarations strictly This patch introduces "_parsealiasdecl" to parse alias declarations strictly. For example, "_parsealiasdecl" can detect problems below, which current implementation can't. - un-closed parenthesis causes being treated as "alias symbol" because all of declarations not in "func(....)" style are recognized as "alias symbol". for example, "foo($1, $2" is treated as the alias symbol. - alias symbol/function names aren't examined whether they are valid as symbol or not for example, "foo bar" can be treated as the alias symbol, but of course such invalid symbol can't be referred in revset. - just splitting argument list by "," causes overlooking syntax problems in the declaration for example, all of invalid declarations below are overlooked: - foo("bar") => taking one argument named as '"bar"' - foo("unclosed) => taking one argument named as '"unclosed' - foo(bar::baz) => taking one argument named as 'bar::baz' - foo(bar($1)) => taking one argument named as 'bar($1)' To decrease complication of patch, current implementation for alias declarations is replaced by "_parsealiasdecl" in the subsequent patch. This patch just introduces it. This patch defines "_parsealiasdecl" not as a method of "revsetalias" class but as a one of "revset" module, because of ease of testing by doctest. This patch factors some helper functions for "tree" out, because: - direct accessing like "if tree[0] == 'func' and len(tree) > 1" decreases readability - subsequent patch (and also existing code paths, in the future) can use them for readability This patch also factors "_tokenizealias" out, because it can be used also for parsing alias definitions strictly.