Added fast path to parser for simple name lookup paths

[mchaput]

This patch adds a "fast path" that bypasses the parser and constructs the path directly when it's just a series of named field lookups (e.g. 'foo.bar.baz').

In my tests this gives a ~2500x (!) speedup for parsing lots of simple paths of this type, which are common in our workflows.

Here's my simple test code:

domain = ("alfa", "bravo", "charlie", "delta", "echo", "foxtrot", "golf",
          "hotel", "india", "juliet", "kilo", "lima", "mike", "november",
          "oscar", "papa", "quebec", "romeo", "sierra", "tango")

def random_path(length: int) -> str:
    parts = [random.choice(domain) for i in range(length)]
    return ".".join(parts)

def test_jsonpath(inputs: list[str]) -> float:
    t = perf_counter()
    for p in inputs:
        p = random_path()
        jp = parse_jsonpath(p)
    t = perf_counter() - t
    print(f"{t:0.4f} seconds")
    return t

if __name__ == "__main__":
    inputs = [random_path(4) for _ in range(100)]
    test_jsonpath(inputs)

[mchaput]

Nevermind, there a couple of problems with the regex (e.g. should end with $, and the key matching can be more liberal).