math/big: add benchmark of Exp with large modulus

math/big: add benchmark of Exp with large modulus

Setting up for improving even modulus.

name                   time/op
ExpMont/Odd-16          240µs ± 2%
ExpMont/Even1-16        757µs ± 3%
ExpMont/Even2-16        755µs ± 1%
ExpMont/Even3-16        771µs ± 3%
ExpMont/Even4-16        775µs ± 3%
ExpMont/Even8-16        779µs ± 2%
ExpMont/Even32-16       778µs ± 3%
ExpMont/Even64-16       774µs ± 6%
ExpMont/Even96-16       776µs ± 4%
ExpMont/Even128-16      764µs ± 2%
ExpMont/Even255-16      761µs ± 3%
ExpMont/SmallEven1-16  45.6µs ± 1%
ExpMont/SmallEven2-16  44.3µs ± 2%
ExpMont/SmallEven3-16  44.1µs ± 5%
ExpMont/SmallEven4-16  47.1µs ± 6%

name                   alloc/op
ExpMont/Odd-16         2.53kB ± 0%
ExpMont/Even1-16       2.57kB ± 0%
ExpMont/Even2-16       2.57kB ± 0%
ExpMont/Even3-16       2.57kB ± 0%
ExpMont/Even4-16       2.57kB ± 0%
ExpMont/Even8-16       2.57kB ± 0%
ExpMont/Even32-16      2.57kB ± 0%
ExpMont/Even64-16      2.57kB ± 0%
ExpMont/Even96-16      2.57kB ± 0%
ExpMont/Even128-16     2.57kB ± 0%
ExpMont/Even255-16     2.57kB ± 0%
ExpMont/SmallEven1-16    872B ± 0%
ExpMont/SmallEven2-16    872B ± 0%
ExpMont/SmallEven3-16    872B ± 0%
ExpMont/SmallEven4-16    872B ± 0%

name                   allocs/op
ExpMont/Odd-16           21.0 ± 0%
ExpMont/Even1-16         24.0 ± 0%
ExpMont/Even2-16         24.0 ± 0%
ExpMont/Even3-16         24.0 ± 0%
ExpMont/Even4-16         24.0 ± 0%
ExpMont/Even8-16         24.0 ± 0%
ExpMont/Even32-16        24.0 ± 0%
ExpMont/Even64-16        24.0 ± 0%
ExpMont/Even96-16        24.0 ± 0%
ExpMont/Even128-16       24.0 ± 0%
ExpMont/Even255-16       24.0 ± 0%
ExpMont/SmallEven1-16    16.0 ± 0%
ExpMont/SmallEven2-16    16.0 ± 0%
ExpMont/SmallEven3-16    16.0 ± 0%
ExpMont/SmallEven4-16    16.0 ± 0%

Change-Id: I5278378b4209a97b16273be581533310e0e4110b
Reviewed-on: https://go-review.googlesource.com/c/go/+/420896
Reviewed-by: Roland Shoemaker <roland@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>