Better separation between running on host and on vm code

[jellonek]

Right now we have mixed code with bad separation between targets, which leads to issues like: #215 or (tested by @poszu on windows):

  = note:    Creating library D:\a\athena\athena\target\release\deps\athena_vmlib.dll.lib and object D:\a\athena\athena\target\release\deps\athena_vmlib.dll.exp␍
          libathena_vm-60dbed9579a2451d.rlib(athena_vm-60dbed9579a2451d.athena_vm.8053e8118ec650d7-cgu.0.rcgu.o) : error LNK2019: unresolved external symbol _end referenced in function _ZN81_$LT$athena_vm..heap..SimpleAlloc$u20$as$u20$core..alloc..global..GlobalAlloc$GT$5alloc17h0620a1c26337143bE