Add collection builder that deduplicates on insertion

[sethrj]

This is intended for use primarily with ORANGE to essentially "compress" parameter data while it's being assembled. It hashes every call to insert_back and returns a previously inserted range if available. There are a few subtleties:

1. The data cannot be modified after it's inserted. The CDF renormalization in physics violates this.
2. Each element must be "uniquely representable", i.e. it can't have padding nor contain floats. Specializations of std::hash must be created for each one.

[amandalund]

Looks good @sethrj!