limb: always inline bitand

Previously, rustc didn't inline `Limb::bitand` inside functions
such as `UInt::add_mod` and `UInt::sub_mod`, when calling them from
a different crate. This is probably because non-generic functions
don't get inlined across crates (unless using link-time
optimization).

src/limb/bit_and.rs

@@ -5,6 +5,7 @@ use core::ops::BitAnd;
 
 impl Limb {
     /// Calculates `a & b`.
+    #[inline(always)]
     pub const fn bitand(self, rhs: Self) -> Self {
         Limb(self.0 & rhs.0)
     }
@@ -13,6 +14,7 @@ impl Limb {
 impl BitAnd for Limb {
     type Output = Limb;
 
+    #[inline(always)]
     fn bitand(self, rhs: Self) -> Self::Output {
         self.bitand(rhs)
     }