Identifiers equal to integer > max_int are not accepted in terms

[fblanqui]

symbol 1000000000000000000000000000 : TYPE;
symbol foo: 1000000000000000000000000000;
[/home/blanqui/src/lambdapi/tmp/toobig.lp:2:12-40] Too big number (max is 4611686018427387903).

This is a problem in the parser. In terms, natural numbers are parsed as P_NLit.
To fix this, one should convert identifiers to P_NLit's at scoping time (instead of at parsing time).
To get around this, enclose big numbers in {|...|}.

[fblanqui]

Fixed in #1182 as desugaring is now defined by recursion on the string: there is no conversion to int anymore.
Note however that integers are not allowed as identifiers anymore.