Implement most semantics for value functions and intensions;
Improve base functions.
Caveats:
Environment is saved in abstraction at the moment of evaluation while it should be saved at the moment of definition;
Recursive functions are not working yet (not sure if they were working in pull request #41);
Handling of hidden dimensions will need more review (in particular re value and intension abstractions), considering that the environment is supposed to contain intensions.
Caveats:
This pull request depends on pull request #41.