`ref` parameters, arguments, returns and `let` returns

[josh11b]

• A parameter binding can be marked ref instead of var or the default. It will bind to reference argument expressions in the caller and produces a reference expression in the callee.
  • Unlike pointers, a ref binding can't be rebound to a different object.
  • This replaces addr, and is not restricted to the self parameter.
  • A ref binding, like a let binding, can't be used in fields of classes or structs.
  • When calling functions, arguments to non-self ref parameters are also marked with ref.
• The return of a function can optionally be marked ref, let, or var. These control the category of the call expression invoking the function, and how the return expression is returned.
  • These may be mixed for functions returning parens or brace forms.
• Any parameters whose lifetime needs to contain the lifetime of the return must be marked bound.
• The address of a ref binding is nocapture and noalias.
• We mark parameters of a function that may be referenced by the return value with bound.