Expose functionality to only build a binary `.gp` codelet, without adding it to a graph

[giordano]

Currently we only provide the macro @codelet to build the .gp codelet and add it to a graph, but it was suggested that it can be useful to just compile the .gp file, outside of a specific program. We should then expose this functionality: we already do that under the hood, we only need to reorganise the code and the API to allow users to only get the .gp file.