Function closure

[lucafavatella]

Main changes:

• Close functions defined in the body of a wherevar
• Close anonymous functions and intension abstractions
• Evaluate arguments of abstractions in the correct environment
• Improve tests

This pull request brings in meck as a dependency.

[lucafavatella]

Pull request updated merging recent master in it, and fixing failing test caused by changes in primops.

[lucafavatella]

Tests have been greatly improved and clarified in pull request #50.

Probably the most efficient way of reviewing this pull request #45 is reviewing it together with pull request #50, in particular:

• Review the changes in src/ in pull request #45;
• Review the changes in src/ in pull request #50;
• Review the tests as found in pull request #50.