Replace checker decls with stdlib

[TristonianJones]

In replacing the checker declarations with the stdlib declarations, a few
small changes have been made which are important to type-checker and
runtime stability.

• decls.Overload instances are collected to Binding() and to []*exprpb.Decl
  instances in the order they are declared.
• Merging two decls.Function will preserve overload ordering, with the function
  on the left of the merge taking precedence over the right.
• Marking a function declaration as disabled or enabled will be preserved across
  function sets
• Expecting type-guards on a subset of overloads will result in type-guards being
  generated for the entire overload set.

This change also deprecates references to the checker.StandardFunctions() and
checker.StandardTypes().