Improves the VERIFY_CHECKs in all _scalar_get_bits_limb32 and _scalar_get_bits_var methods.
The initial prompt was noticing that scalar_4x64_impl/secp256k1_scalar_get_bits_limb32 was not restricting to 32-bit limbs correctly. Then missing range checks for offset were added and all such checks rewritten to avoid overflow.
With these changes, the _low and _4x64 impls of _get_bits_var can no longer forward to _get_bits_limb32, so those calls were inlined instead.