Spans! (Closes #39)

[SnirkImmington]

This PR rewrites the AST to use a Span type instead of occasionally storing tokens!

• Adds Location and Span in lex
• Removes some of the last of the public struct fields
• All AST nodes have a Span instead of occasionally having tokens, and are sometimes created with a start Location (+ children's ending)
• Checker errors use Spans instead of a source Tokens
• Identify errors can include Span info of previous variable definitions