Stdlib: Removed all references to String constructors in nit stdlib and tests.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
nitg: Modified compilation routine to avoid use of String contructors

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
nitc: Removed use of String::with_native in nitc toolchain

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
c_src : Regenerated c_src to prevent use of String contructors

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
stdlib: Removed constructors from String

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>