Make the mul tables be calculated at compile time with constexpr

6 jobs for constepxr_decrupt in 4 minutes and 54 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #263176
build

00:03:33

passed #263181
build_android

00:02:24

passed #263177
build_clang_libcpp

00:03:32

passed #263180
build_clazy

00:04:53

passed #263179
build_mingw64_fedora

00:03:26

passed #263178
build_ubuntu_16_04

00:01:51