Compiler is now swiftc
Rework declaration modifiers

Make declaration modifiers and access control work better, and work
together properly.

Support the new `dynamic` access control modifier.

Support the new infix/prefix/postfix declaration modifiers and operator
declaration syntax.
Handle mutating/nonmutating properly on computed vars
Merge branch 'beta5'

* beta5:
  Handle mutating/nonmutating properly on computed vars
  Rework declaration modifiers
  Compiler is now swiftc