Specify compile-time evaluation of field and index accesses

[jaehyun1ee]

This adds to the list of (local) compile-time known values in section 18.1. (#1323)

In summary, for index access,

• e1[e2] is local compile-time known if both e1 and e2 are local compile-time known.
• e1[e2] is compile-time known if both e1 and e2 are compile-time known.
• Note that e1 should be of a header stack or tuple type.

And for field access,

• e.x is local compile-time known if e is local compile-time known.
• e.x is compile-time known if e is compile-time known.
• Note that e should be of a struct, header, or header union type.