expose more knobs

[BurntSushi]

Regex currently has two constructors, Regex::new and Regex::with_size_limit. The latter is the same as the former, except it allows one to bound the size of the compiled program. The idea is here to control how much memory is used if a regex is compiled from an untrusted source.

There are other knobs that seem useful for exposing to the user:

• The recursion limit used in regex-syntax for recursively simplifying a regex. (It'd be nice to move this to a stack on the heap, but it seems tricky.)
• The cache size used in the lazy DFA. For regexes that create lots of distinct states, it's possible to realize big gains if you're willing to spend the memory to do it. Currently, it is set to a constant of ~2MB.

There are other knobs that maybe shouldn't be exposed, but could:

• Control the budget for extracting literal prefixes.
• Control whether the DFA is used. (There exists regexes and inputs where avoiding the lazy DFA is actually faster, but it's probably hard to know when this is without some experience with the internals of this crate.)
• Control whether common suffixes are factored out of compiled programs. This could reduce compilation times at the expense of bigger programs.

It's not clear what, if any, of these things should be exposed. I feel like the knobs that control memory bounds should be accessible to callers, because when you need them, you really need them.

[BurntSushi]

I've given this some thought, and this is where I'm currently at:

1. There should be a struct with no public fields that permits setting various knobs that can be applied to the construction of a Regex. Knobs can be set using methods. This permits expanding the set of the knobs in the future in an API compatible way.
2. We should probably start with just two knobs: the existing compile size limit and a new knob that controls how much cache the lazy DFA is allowed to use.
3. This probably means that Regex will continue to need an alternate constructor, much like how Vec or HashMap have alternate constructors. For example, Regex::with_config or something.