Add Support for Char Literals

[rcosta358]

This PR adds support for char literals, preventing the exception Literal of type char not implemented when chars are used in the Java source code. In the SMT solver, char literals are translated to their corresponding integer representation.

[CatarinaGamboa]

Great! We should be careful in the future if we want to have more operations with chars inside the refinements, since they can be misleading since we are using only their int values.
If possible add a test with a char being used as an int like

// function
void write(int x){...}
// client
write('c');