In order to not conflict with new error codes that Vim adds, all Neovim
error codes should be above 5000.  The three existing sub-5000 error
codes (E926, E951, and E952) are now E50003, E5004, and E5005
respectively.

E953 was removed in 6167ce6df2753d5474ad49aea19f5957128ab015, so just
remove it from the help.

This ought to prevent stack overflow, but I do not see this actually working:
*lua* code crashes with stack overflow when trying to deserialize msgpack from
Neovim, Neovim is fine even if nesting level is increased 100x (though test
becomes very slow); not sure how recursive function may survive this. So it
looks like there are currently only two positive effects:

1. NULL lists are returned as empty (#4596).
2. Functional tests are slightly more fast. Very slightly. Checked for Release
   build for test/functional/eval tests because benchmarking of debug mode is
   not very useful.

For string() it looks like no optimization, sometimes performance is even worse. 
Since it was designed to avoid heap allocations in clear_tv let’s see whether it 
will make any difference once clear_tv uses typval_encode to avoid stack 
overflow in the disabled test.

Patch made up by oni-link.