v0.3.0 - "Cutting Edge"

New features:

• Named variables and wildcards (#14)
• Integer arithmetic (see logru::resolver::arithmetic module) (#18)
• Extensibility through custom predicate resolvers (see Resolver trait and the REPL example) (#17 and #19)
• Cut (#20)

Major breaking changes:

• Numeric variable naming is no longer supported in surface language.
• Struct CompiledRuleDb is now called RuleSet.
• Module solver is now called search.
• The Universe type is now roughly subsumed by the SymbolStore and RuleResolver types.
• The NamedUniverse type has been absorbed by SymbolStore, which now provides allocating symbol
  IDs and naming them in one.

Bug fixes:

• Occurs check did not follow bound variables (fixed in #16)