Use backtick string literals

This pull request changes the `ToSql` visitor to use backtick quoted string literals instead of constants for static strings.

I've explained most of it by updating the massive comment at the top of the file, but the main benefit to using backtick quoted string literals instead of constants is code locality. Although a slight method call overhead is incurred, we still retain the benefit of not allocating lots of objects.

I've also converted a bunch of static string literals in the file that weren't referring to one of the constants to backtick strings, so hopefully this PR also brings a nice perf improvement as well as a huge readability boost.