External contract member checks

(from #72)

External contracts can be specified with the external trait syntax:

external trait Ext {
  func someFunc()
  func someReturningFunc(param: Int64, other: UInt72) -> Int
}

External contracts are severely limited in what can or cannot be specified; a lot of normal trait functionality should be disabled.

[x] check that external trait members are only function signatures - no variables, no inits, no fallbacks, no events
[x] check that there are no implementations for functions (signature only!)
[x] check that there are no protection blocks for external traits
[x] check that there are no type states for external traits
[ ] check hash collisions (see note at the bottom of https://github.com/flintrocks/flint/issues/73#issuecomment-437690396)
[x] check that function signatures only use external (Solidity ABI) types - will be done as a part of #90

flintrocks / flint