Rewrite modexp with numext

[mohanson]

Note: Currently I use

numext-fixed-uint = { git = "https://github.com/yangby-cryptape/rust-numext", branch = "fix-div-slow-bug" }

in dependencies because @yangby-cryptape tell me It may take some times to waiting for the fix to merge to the master.

[keroro520]

muta is using numext-fixed-uint = "0.1.2" but here is "0.1.3"

Maybe it is better that cita-vm and muta use the same version of numext-fixed-* ?

[mohanson]

There are some bugs in numext-fixed-uint = 0.1.2, I will update it on muta.

ref: https://github.com/cryptape/rust-numext/issues/36 ref: https://github.com/cryptape/rust-numext/issues/34

[keroro520]

There are some bugs in numext-fixed-uint = 0.1.2, I will update it on muta.

ref: cryptape/rust-numext#36 ref: cryptape/rust-numext#34

Excellent !