Allow loading the address space from various sources

Fixes #438 (closed) .

The configuration of the embedded address space is still a bit of a mess at the moment: we can either compile it in without using the "loader" API, which is what everything does except toolkit_test_server (in which case the ADDSPACEGEN_DIR CMake variable applies), or use the embedded address space loader (in which case the EMBEDDED_ADDRESS_SPACE_FILE CMake variable applies). Once we reach agreement on this MR, I'll port the tests + benchmarks etc. to use the loader API.