[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.