Duduplicate character literal variables and RT_USE_LOOKUP_TABLE lookup table

[laurenthuberdeau]

Context

When using RT_USE_LOOKUP_TABLE, variables of the form __c2i_{char}={character code} are created for all alphanumerical characters. This is redundant since pnut already creates variables for character literals. When using RT_USE_LOOKUP_TABLE, all character alphanumerical literals should get variables and these variables should be used instead of having these separate __c2i_{char} variables.