OWL literal values

[timothee-haudebourg]

This branch introduces a better support for OWL (and XSD) literal values, which means:

• Undestanding the semantics and relations of/between each OWL datatypes (which include XSD datatypes).
• Adding a syntax for literal values.

This will allow TreeLDR to:

• Define enumeration types (owl::oneOf) which enumerates all the instances of a type (that's why we need to be able to express values).
• Define value restrictions both in the compiler (already partially implemented) and using the TreeLDR syntax (not implemented yet).

Todo

• Add a Value type.
• Add an Enumeration type
• Allow attaching a value to an enumeration layout variant (so we can represent an enum type with an enum layout)
• Add a singleton layout (so that in the special case of a enum layout with a single variant, the name of the variant may be omited)
• Support every XSD lexical representations for the values (implemented in the xsd-types crate).
• Syntax for literal values
• Syntax for enumeration type/layout
• Syntax for datatype restrictions