templater: introduce one-pass parsing of nested template strings Instead of re-parsing quoted strings as templates, the tokenizer can delegate the parsing of nested template strings to the parser. It has two benefits: 1. syntax errors can be reported with absolute positions 2. nested template can use quotes just like shell: "{"{rev}"}" It doesn't sound nice that the tokenizer recurses into the parser. We could instead make the tokenize itself recursive, but it would be much more complicated because we would have to adjust binding strengths carefully and put dummy infix operators to concatenate template fragments. Now "string" token without r"" never appears. It will be removed by the next patch.