Introduce Inflectable and InflectableTerm

These are the basis of a new system for inflecting strings by gender.
Add InflectableTerm constants for English pronouns
Make Inflectable public
Add InflectableString type

Represents a series of Inflectable instances which can be inflected together.
Add support for InflectableString literals

Lets you construct a string from Inflectable objects, like InflectableTerm.
Add front-end inflect() functions
Move conformances into Genderable.swift
Remove unneeded array

There's a variadic form of this too.
Merge branch 'feature/inflectable-string' into develop