v0.4.0 - "Concurrent Queries"

Many thanks to @dcz-self for a bunch of contributions!

New features:

• The textual language now supports line comments with % (#26)
• VarScopes now provide additional functions to inspect the variables in the scope (#23)
• TextualUniverse is now captured immutably by prepared queries, so that multiple queries can be run concurrently against the universe (#27)

Breaking changes:

• SolutionIter now returns Solutions rather than Vec<_>s, making it easier to relate goal variables to solution terms (#25)
• The concrete symbol storage used by e.g. the parser and some resolvers is now abstracted behind a SymbolStorage trait (as part of the works for #27)