Load the guest binary once and map into each Sandbox read-only

[simongdavies]

Each time a sandbox is created we allocate memory for the guest binary and then copy the binary into memory. It should be possible to do this once and then map that memory from the host into each Sandbox.

We will need to do more parsing of the guest binary that at present to take account of writeable sections of the binary , for example relocations will need to be provided for each sandbox if and when we have variable load addresses. Any writeable state in the binary will need to also be taken account of (maybe by implementing COW in the guest).