Change build system to use Cargo.

I moved all the sources to src/
renamed the projet and the binary to r7_rs_VM du to the impossibility to use
a dot in a project name with Cargo.

Becarefull it does dont build with the https://github.com/thestinger/rust-gmp
but it will in a few time a pull-request is on it on for patching the
dependancies https://github.com/thestinger/rust-gmp/pull/16