Possible inlining improvements

I ran RUSTFLAGS=-Ccodegen-units=1 cargo bench to see what changed versus just cargo bench. There were a few that stuck out in particular:

• wrapping ops/widening_mul
  • I512xI512: -25%
  • I1024xI1024: -4%
• widening ops/concatenating_mul
  • I128xI128: −18%
  • I512xI512: −31%
• wrapping ops/sub
  • I512-I512: −35%
  • I1024-I1024: −25%
• bounded random/random_mod
  • U1024: -11%
  • U1024 tiny high limb: -9%
• wrapping ops/widening_mul
  • U256xU256: -12%
• wrapping ops/mul_mod_special
  • U256: -10%
• extended greatest common divisor/xgcd
  • 1: -3%
  • 2: -3%
  • 3: -2%
  • 5: -23%
  • 6: -16%
  • 7: -18%
  • 8: -5%
  • 16: -22%
  • 32: -16%
  • 128: -8%
  • 256: -5%
• left shift/shl_vartime
  • small, U2048: -40%
  • large, U2048: -41%
• left shift/shl_vartime_wide
  • large, U2048: -22%
• right shift/shr
  • U2048: -8%
• right shift/shr_vartime_wide
  • large, U2048: -24%
• modular ops/invert_mod2k_vartime
  • U256: -18%

cc @andrewwhitehead @erik-3milabs

Start by running RUSTFLAGS=-Ccodegen-units=1 cargo bench and compare it with cargo bench, focusing on the listed benchmark groups such as widening_mul, shl_vartime, xgcd, and invert_mod2k_vartime. Trace those operations to their implementations and benchmark coverage; done means identifying and addressing justified inlining improvements while confirming the relevant benchmark results.