[bug] implement parse failures

[evolbug]

Several failures:

• block is ignored if there is a keyword between two user types

  Test: struct {}
  implement Test if Test {}

• parse fails if there's a newline in implement block body but no method

  Test: struct {}
  implement Test: Test {
  }

• implement accepts Type in place of Trait

  Test: struct {}
  implement Test: Test {
  foo: fun() {}
  }

• parse fails if there's extraneous newlines after method

  
  implement Data: Data_T {
  clone:          extern fun(self) -> Data

}

[nilq]

Yessir.