Add missing bits for supporting default in executable specs.

[yav]

Fixes #813. The changes are:

• Replace the todo exception, with a call to the relevant default function.
• Remember to add the records from default to the record map.

Note: some future refactoring could be useful, but is not pressing, to factor out common stuff for calling default functions.

[septract]

This works on the instances of default in the MKM proofs, nice! I should note that we only use default as a placeholder where we need an arbitrary value of the type, which is then thrown away. So I haven't tested whether default is correctly implemented, just that cn test can handle the construct at all.