Sketch for a more general Pattern API

Presumably intended purpose of a more general Pattern API:

• searching through a 1-dimensional, slice-like data structures in a linear fashion
  • means we can use a model close to slices, or C++ iterators
• compatibility with iterator model
  • lazy progression possible
  • possibly double ended
• usable with mutable data
• should yield matches with minimum post-processing or bound checks necessary
• should be able to give position values relative to start of haystack
• Reusing implementations across the API
  • Needs abstract interface independend from slice type
  • Means being able to write a generic Split or Find function

Possible extensions, but probably out of scope:

• searching through arbitrary n-dimensional data structures
  • would mean need for arbitrary search position type

Result of iterativly making the design more general

Each has an implementation in a ::vN submodule

• v1: str-only shared references with (usize, usize) search slice
  • Current Pattern API.
• v2: arbitrary slice type, shared references with (usize, usize) search slice.
• v3: arbitrary slice type, shared references with associated U search slice.
  • Allows replacing (usize, usize) with something more low level like (*const u8, *const u8).
  • (not impld): seperate Pattern API for mutable slices
    • Would require changes to haystack accessor, since there'd be mutable aliasing otherwise.
• v4: arbitrary Ptr<slice> type allowing mutability, with associated U search slice.
• v5: arbitrary Ptr<slice> type allowing mutability, with refactored associated types that work more similar to pointers / C++ iterators.