Add support for :literal_equality

This allows Enumerables to decline to preserve term-inequality for
literals that are value-equal.

For example: `"01"^^xsd:integer` and `"1"^^xsd:integer` have equal
values, but are not equal per RDF term-equality. An enumerable
supporting `:literal_equality` must preserve this distinction across
reads; one that does not may canonicalize the lexical representation of
its literals at its own discretion.

Note that an `Enumerable` need not be `Mutable` to perform this
canonicalization. For example, a data store may have an "optimize"
operation which performs the canonicalization independent of RDF.rb's
interactions.

Author: Tom Johnson

lib/rdf/mixin/enumerable.rb

@@ -68,14 +68,15 @@ module Enumerable
     # Supported features include:
     #   * `:graph_name` supports statements with a graph_name, allowing multiple named graphs
     #   * `:inference` supports RDFS inferrence of queryable contents.
+    #   * `:literal_equality' preserves [term-equality](https://www.w3.org/TR/rdf11-concepts/#dfn-literal-term-equality) for literals. Literals are equal only if their lexical values and datatypes are equal, character by character. Literals may be "inlined" to value-space for efficiency only if `:literal_equality` is `false`.
     #   * `:validity` allows a concrete Enumerable implementation to indicate that it does or does not support valididty checking. By default implementations are assumed to support validity checking.
     #   * `:skolemize` supports [Skolemization](https://www.w3.org/wiki/BnodeSkolemization) of an `Enumerable`. Implementations supporting this feature must implement a `#skolemize` method, taking a base URI used for minting URIs for BNodes as stable identifiers and a `#deskolemize` method, also taking a base URI used for turning URIs having that prefix back into the same BNodes which were originally skolemized.
     #
     # @param  [Symbol, #to_sym] feature
     # @return [Boolean]
     # @since  0.3.5
     def supports?(feature)
-      feature == :validity
+      feature == :validity || feature == :literal_equality
     end
 
     ##