Modify type checker. Now, it only accepts builtin values that have been declared in the environment. The idea is to be able to track which classes of builtin values have been used in a given environment. We want to be able to quantify the size of the trusted code base for a particular development.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Modify environment. Now, when a builtin value is declared, if it has a unicode alternative representation, then we add it as a definition. Now, everything that occurs in the environment has been 'declared'.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Add unicode name for the types: Nat, Int and Real

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>