Allow directly nested arrays of arrays

[dnwpark]

Allows directly nested arrays.

The query select ([[1],[2]]++[[3]])[1:3][0]; returns [2].

The query select find(array_replace([[1],[2],[3]],[2],[4]),[1]); returns 0.

[dnwpark]

Needs lots of tests, especially for the schema.

While compiler can handle nested arrays, the protocol can't

_localdev:main> select [[1]];
edgedb error: ProtocolEncodingError: array shape is invalid

array_unpack does not work because I need more special handling for set returning functions.

array_fill doesn't work because return type record[] is not supported for SQL functions.