Redefining a symbol should clear its immutbale bit

[jpellegrini]

set! is not allowed on a mutable binding; but a 'define' would actually redefine the binding, so we should clear the immutability bit. In order to forbid defines, one should make the module immutable (not the symbol) -- which is already possible using module-immutable!. Right?

[egallesio]

You are right! Thanks for the fixed. Merged