Implemeted the basic allocation logic in jem_alloc.
Implemeted the basic allocation logic in jem_alloc.

Memory is organised as a linked list of blocks, each with a header
consisting of an 'allocated' flag, and the block size. This avoids
the need to maintain a list of freed blocks.

jemtucker/opsys,jemtucker/opsys,jemtucker/opsys