Allow visiting protolists

[psybers]

before e: Expression -> {
    visit(e.expressions);
}

gives an error:

t.boa: compilation failed: Encountered typecheck error at line 6, columns 8-12. no such function visit([protolist of Expression])
        visit(e.expressions);
        ^^^^^

This should be supported. It is essentially the same as the following:

before e: Expression -> {
    foreach (i: int; e.expressions[i])
        visit(e.expressions[i]);
}