Functions with literals and params should type resolve

[tobyhede]

SQL statements containing literals or param placeholders currently trigger type check error.

-- The arg `'x'` causes mapping to fail.
-- This causes `encrypted_text` to fail to decrypt in the results.
select concat(plaintext, 'x'), encrypted_text from encrypted;

This change unifies function types with Type::any_native().
EQL values will be rejected in function calls, which is the correct behaviour in most situations as EQL columns need to be decrypted.

eg, concat(encrypted_column, 'str') will attempt to concat the encrypted json data.

Future work will add support for the EQL functions.

[CDThomas]

Had a couple of minor comments around some unify calls that look redundant, but LGTM otherwise.