Types

[djluck]

Adding stronger validation of PromQL expressions, principally type checking which will allow consumers to validate the type an expression produces.

This has meant a lot of improvements:

• Aggregate operator and function signatures are now defined in the library and parameter counts and types can be checked.
• Binary expressions are now parsed with the (correct) associativeness: left associativeness.
• Binary operators now have precedence associated with them and binary expressions are grouped according to this
• Every expression can determine it's return type
• Expression types preceeding offset expressions are now validated more carefully
• Positions of parsed expressions from the source input are now available in all AST nodes