The algorithm now checks for invalid UTF-8 sequences, which is required
by the Go spec.
This gets the tests of the unicode/utf8 package to pass.
Also add bytes.Equal for Go 1.11, which again is necessary for the
unicode/utf8 package.
Previously, we implemented individual bytealg functions via linknaming, and had to update them every once in a while when we hit linker errors.
Instead, this change reimplements the bytealg package in pure Go.
If something is missing, it will cause a compiler error rather than a linker error.
This is easier to test and maintain.
strings.IndexByte was implemented in the runtime up to Go 1.11. It is
implemented using a direct call to internal/bytealg.IndexByte since Go
1.12.
Make sure we remain compatible with both.
