Support dumping empty lists with type annotation

[s-zeng]

Dumping an empty list currently gives a TypeError. We should support a mechanism to allow empty lists to be tagged with a type.

Example: Currently, the following fails with a TypeError:

>>> dhall.dumps([[3, 4], []])
TypeError: cannot serialize value without a type annotation: List([])

... but the following works just fine:

>>> dhall.dumps([[3, 4], [5]])
'[[3, 4], [5]]'

To be consistent, we could support type inference in homogeneous lists to add a type annotation to the empty list