Move runtime/base/server to runtime/server

This is an incremental step towards moving it all the way
to hphp/server.  This flattens base but doesn't untangle
the server files from lib_hphp_runtime
Make Asm::code private.

Having everybody and their uncle reading and writing fields out
of a.code and stubs.code was making assembler work hard. This replaces most
reads with accessors, and all writes with structured friends of
X64SAssembler.