Optimization: Check for implicit anchor

[Andersama]

Slight improvement on #95

From O'Reilly's book:

An engine with this optimization realizes that if a regex begins with .* or .+ and has no global alternation an implicit ^ can be prepended to the regex.

Intuition is that .* and .+ collide / consume all the initial characters that don't really match the pattern ergo .* and .+ will consume the entire input string in one go...bumping the initial start position is much like remaining in the .* or .+

[Andersama]

Example: https://gcc.godbolt.org/z/3WvT98bfE

[Andersama]

In testing without the implicit anchor searching for .*pattern I could not get the benchmark to complete (there's excessive amount of backtracking here). With this check however it seems to run roughly half as well as just searching for pattern directly. It's probably worth extracting the .* from the pattern and running search directly.