Make separate spec cases for filter, map, etc. to handle an empty sequence

[elenam]

Babel specs for filter, map, and possibly other functions allow the first argument to be any to allow the case of an empty sequence (legal in Clojure):

(filter 5 [])

This returns an empty sequence.

However, it leads to a ClassCastException error, not a spec error, on the first argument not being a function when the sequence is not empty, for example:

(filter 5 [1])

We need a special spec case for an empty sequence since otherwise error messages for a very common error are not sufficiently descriptive.

[elenam]

There are two different problems here (and one more that we discovered upon more exploration):

1. An empty sequence passed to filter is legal Clojure code regardless of the first argument. This is also the case for map and some other functions.
2. The spec for filter is wrong and is causing a wrong error message for a non-empty sequence when the first argument is not a function.
3. There is a related issue with a spec for map which allows anything for the first argument in the general case, but it it should be done only for an empty sequence. This spec also needs to be rewritten. This is not covered in this issue, see Spec predicates for map with at least one empty sequence #85

[elenam]

The case of map is more complex than the other functions since map takes an arbitrary number of sequences. I separated it into a different issue: #85