So far `std.jsonnet` needed to be updated seprately from
the cpp-jsonnet submodule. Since we should update it anyway
at the same time (to make sure the tests are running fine),
we can just as well get it directly from there, eliminating
the extra step of copying the new `std.jsonnet` version.
This change updates the cpp-jsonnet used.
Add go mod
Also make the build docs nicer and cleanup the file directory to be in line
with most other go projects. This also make it so you can build jsonnet
without setting -o on go build.
