Simplify proposal, specify eager evaluation

[rbuckton]

This includes the following changes:

• Partial application now specifies eager evaluation.
  • Arrow functions already provide lazy evaluation.
  • Eager evaluation sidesteps f({ x: ? }) as sub-expressions cannot be eagerly evaluated.
• Partial application can no longer be used with new.
• Drops the ... placeholder token, though this may be revisited in a later proposal.
• Clarifies behavior of f(g(?)).
• Clarifies that ? is not an expression, merely an extension of the ArgumentList production.
• Clarifies that x + ? (or use of ? in any other arbitrary expression) is not supported.
• Clarifies that ?.f() is not supported.
• Clarifies that f(x + ?) is not supported.

The goal of these changes is to make the proposal more focused and define explicit behavior.

Fixes: #1, #2, #6, #9, #11, #14, #18, #20

[rbuckton]

//cc: @littledan, @gilbert, @royling