specify MSRV

[joseluis]

Currenly only the edition 2018 is specified, which implicitly sets the MSRV to 1.31.0, but libm doesn't compile with that version.

The first compatble rust version is 1.43.0, when the associated EPSILON constant was stabilized, which is used in the rint / rintf modules.

And it also fixes another compilation error while trying to load the limb_helper module.