Package-manage GMP

[battlmonstr]

Currently there are 2 package managers:

• git + cmake (via git submodules and cmake scripts)
• hunter

GMP is not included there, and has to be installed manually.

It would be nice to integrate it into the common setup by either:

• checking it to git, having a submodule and a related cmake script
• or making a hunter script wrapping it, and bringing it using hunter

[AndreaLanfranchi]

Actually package managers are 3 : there is also vcpckg for windows (is needed to install and use MPIR instead of GMP on windows)

[canepat]

Closed by #1150 and #1184