Lower MemRef GetGlobal and write data to json files

[jiahanxie353]

This patch lowers memref::GetGlobalOp and write data that it carries to corresponding json files, so that they can be used later by Calyx. Json data will be written to the same directory as the source file. Since there might be situations where users don't wish to do so, writing to json will only be carried out if users specify --lower-scf-to-calyx="write-json=true".