Flexible solver/scrambler lifetime management

[lgarron]

For the commandline, FFI, and most basic use cases, a lazy static "singleton" for each scrambler is probably fine:

twsearch/src/rs/scramble/puzzles/square1/scramble.rs

Line 52 in c25a6c7

static ref SQUARE1_SOLVER: Mutex<Square1Solver> = Mutex::new(Square1Solver::new());

However:

• We may want to provide an API call to deallocate a given scrambler if the caller would rather free the resources.
• We need to expose an API to provide access to the scramblers themselves, so the caller can e.g. configure them, run them in parallel/interleave them/run them on different threads.

Being able to free all (well, almost all) memory after generating scrambles is also a good litmus that we have a good, safe architecture.