Fixes:
1. Allocate space for the NUL byte.
2. Do not exclude last line from range.
3. Remove code for sandbox: it is handled earlier.
4. Fix index in new_line_transformed when converting NULs to NLs.
5. Always allocate new_line_transformed, but save allocated value.
1. Use `char *` for strings.
2. Add `const` qualifiers.
3. Add attributes and documentation.
4. Handle skipping *inside*.
5. Handle non-heredoc argument also inside: deferring this to the caller is
pointless because all callers need the same thing. Though new ex_lua caller
may live without allocations in this case, allocating nevertheless produces
cleaner code.
6. Note that all callers call script_get with `eap` and `eap->arg`. Thus second
argument is useless in practice: it is one and the same always and can be
reached through the first argument.
No tests yet, no documentation update, no :lua* stuff, no vim module.
converter.c should also work with typval_T, not Object.
Known problem: luaeval("1", {}) results in
PANIC: unprotected error in call to Lua API (attempt to index a nil value)
Ref #3823
