Translation of highlighter.c into Go

This repo contains highlighter.c and highlighter.go. The latter is a translation of the former. As a result of being translated from C, it contains two functions -- append and at -- that make use of unsafe operations. Additionally, the entry function appends a NULL byte, which results in an unfortunate allocation. It does work, however, and is decently fast.

In order to test equivalence, there's a simple fuzz test, runnable with go test -v -fuzz FuzzComparison using Go ≥1.18.

The goal here is to get rid of the unsafe operations in highlighter.go and to get rid of the NULL byte addition.

It would be nice to do this with as few changes as possible. For example, functions like isCaselessSame or isValidIPv6 may have similar functions in the standard library, but those might possibly behave differently in subtle ways. Rather than trying to rewrite everything, it is preferable to simply work on replacing the bespoke representation of strings and string slices, in order to remove the unsafe operations and get rid of the NULL byte addition. Additionally, keeping the structure close to the original C implementation makes it easier to maintain moving forward.