This PR defines `String.Slice.replace` and redefines `String.replace` to
use the `Slice` version.
The new implementation is generic in the pattern, so it supports things
like `"education".replace isVowel "☃!" = "☃!d☃!c☃!t☃!☃!n"`. Since it
uses the `ForwardSearcher` infrastructure, `String` patterns are
searched using KMP, unlike the previous implementation which had
quadratic runtime. As a side effect, the behavior when replacing an
empty string now matches that of most other programming languages,
namely `"abc".replace "" "k" = "kakbkck"`.
This PR ensures that searching for an empty string returns the expected
pattern of alternating size-zero matches and size-one rejects.
In particular, splitting by an empty string returns an array formed of
the empty string, all of the string's characters as singleton strings,
followed by another empty string. This matches the [Rust
behavior](https://doc.rust-lang.org/std/primitive.str.html#method.split),
for example.
This PR fixes a bug in `String.Slice.takeWhile` which caused it to get
its bookkeeping wrong and panic. The new version only uses safe
operations on `String.Slice.Pos`.
This PR shows that the iterators returned by `String.Slice.split` and
`String.Slice.splitInclusive` are finite as long as the forward matcher
iterator for the pattern is finite (which we already know for all of our
patterns).
At actually also completely redefines the iterators to avoid the inner
loop in `Internal.nextMatch` which generates inefficient code. Instead,
when encountering a mismach from the matcher, we `skip` the split
iterator.
