Improve implementation of Association and associated functions

[mmatera]

• Association expressions should be stored as a special class of expressions, for faster access to their key/val elements.
• The implementation of Map related functions acting on Association expressions should be improved to reproduce the behavior in WMA (see comments and discussions in Handle RuleDelayed in Map over Association expressions #1694). For example,

Map[F, Association[a->1,b->2, NotARule]
produces in WMA
Association[F[a->1], F[b->2], F[NotARule]
instead of
Association[a->F[1], b->F[2], F[NotARule]]