Enhancement: Subr

builtin functions definitions have been reworked for the seek of code
performance: 'and' defs have been replaced by 'let' defs.

'eval_c' now takes into account the new fixed arity subr (F1, F2 and Fn which
stands for special forms).
Enhancement: added fixed arity subr

A new 'subr' type has been defined to add fixed arity subr values.

All of the source code has been updated accordingly.
error catching moved to toplevel

Error and Not_found exceptions are now catched at toplevel in place of 'eval'.
interpreter performance improved

Set the '-inline' aggressiveness to 50 which gives slight performance
improvement.