decimal: introduce and use lua_pushdecimalstr()

decimal conversion to string in Lua used decimal_str() function.
The function is not safe to use in preemptive context like Lua,
where any attempt to push something onto the Lua stack might
trigger GC, which in turn might invoke any other code.

It is not safe because uses the static buffer, which is global and
cyclic. Newer allocations can override the old data without any
warning.

The same problem was fixed for tt_uuid_str() and uuids in
box.info in one of the previous commits.

The patch adds a new function lua_pushdecimalstr() which does not
use the static buffer. It is now used to push decimals safely on a
Lua stack.

Follow up #5632
Follow up #6050
Closes #6259