lua/decimal: rework luaT_{new,push}decimal() API

`luaT_pushdecimal()` now accepts a decimal argument to copy into the Lua
managed memory.

`luaT_newdecimal()` now doing what `luaT_pushdecimal()` did before: just
allocates a storage for decimal on the Lua stack.

This naming looks much more friendly. It also seems that it follow Lua
API names: `lua_push*()` accepts what to push, `lua_new*()` doesn't.

A couple of notes around the change:

* On the first glance it seems that `luaT_pushdecimal()` is redundant,
  because it can be written using `luaT_newdecimal()` + copying. That's
  truth in contexts, where we know size of the internal `decimal_t`
  structure. A user of the module API don't know it and should pass
  `box_decimal_t *` pointer to `luaT_pushdecimal()` to write the value.
* I use `memcpy()` instead of just `*a = *b` in `luaT_pushdecimal()` to
  copy the padding byte content. Who knows, maybe this not-so-legal way
  to hold extra information may be crucial for some use case or will
  allow us to add one field into the structure.

This is preparatory commit for exposing `luaT_*decimal()` functions into
the module API.

Next commits will change uuid, datetime, interval functions in the same
way.

Part of #7228

NO_DOC=refactoring, no user-visible changes
NO_TEST=will be tested in a next commit, after exposing to the
        module API
NO_CHANGELOG=refactoring, no user-visible changes