Rename StringExtensions.Replace so they don't conflict with native methods

[scottdorman]

The StringExtensions.Replace methods can potentially conflict with native string methods in .NET 2.1. Specifically, the compiler seems to prefer Replace(String oldValue, String? newValue, StringComparison comparisonType) rather than the Replace(this string source, string oldValue, string newValue, int occurrences) extension when occurrences == 0.

This can be mitigated by renaming the Replace extensions to something more descriptive, like ReplaceOccurrences. It also doesn't really make sense that occurrences is allowed to be 0, as that is effectively a NoOp.