This PR gives a proof of `LawfulToForwardSearcherModel` for `Slice`
patterns, which amounts to proving that our implementation of KMP is
correct.
Note that this PR also changes the KMP implementation to make it
slightly more efficient and easier to verify. I also have a correctness
proof for the old implementation, so there were no bugs in the old
implementation.
This PR adds `Std.Slice.Pattern` instances for `p : Char -> Prop` as
long as `DecidablePred p`, to allow things like `"hello".dropWhile (· =
'h')`.
To achieve this, we refactor `ForwardPattern` and friends to be
"non-uniform", i.e., the class is now `ForwardPattern pat`, not
`ForwardPattern ρ` (where `pat : ρ`).
This PR fixes the KMP implementation, which did incorrect bookkeeping of
the backtracking process, leading to incorrect starting ranges of
matches.
The new implementation does not require `partial` anywhere.
