"Optimize" GCC by removing const

[chfast]

The bug in GCC prevents some optimizations if a local struct variable (like uint256) is const. This PR removes some consts that affects the optimization. See bug report https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106786.

mod<addmod_public>/64                                         -0.1555         -0.1555            38            32            38            32
mod<addmod_public>/128                                        -0.0741         -0.0741            35            32            35            32
mod<addmod_public>/192                                        -0.1834         -0.1834            45            36            45            36
mod<addmod_public>/256                                        -0.1706         -0.1706            40            33            40            33
mod<mulmod>/64                                                -0.1329         -0.1329            55            47            55            47
mod<mulmod>/128                                               -0.1377         -0.1377            62            53            62            53
mod<mulmod>/192                                               -0.1233         -0.1233            77            68            77            68
mod<mulmod>/256                                               -0.1266         -0.1266            80            69            80            69
ecmod<addmod_public>                                          -0.7779         -0.7779            31             7            31             7
ecmod<mulmod>                                                 -0.1339         -0.1339            74            64            74            64
binop<uint256, uint256, public_mul>                           -0.0616         -0.0617             7             7             7             7
binop<uint512, uint256, umul_>                                -0.4201         -0.4201            19            11            19            11

[codecov-commenter]

Codecov Report

Merging #279 (2732d7f) into master (8b5f474) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #279   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines         1941      1941           
=========================================
  Hits          1941      1941           

Flag	Coverage Δ
32bit	100.00% <100.00%> (ø)
gcc	99.47% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
include/intx/intx.hpp	100.00% <100.00%> (ø)