TypedIdentifier

[maciejhirsz]

First batch of changes from the big PR:

• Added TypedIdentifier, following the grammar spec, that must have a type. Function definitions and variable declarations now use Vec<TypedIdentifier> instead of Vec<Identifier>.
• Added From impls to easily create instances of Identifier from string literals ("foo".into() or Identifier::from("foo")).
• Validator impl for Identifier checks that the identifier is in accordance with the grammar.
• Changed the error type from String to &'static str in the validator, although custom enum or using the error_chain crate would be better here I reckon.