The Rust module (see the issue) needs a getter and a setter for decimal
values on the Lua stack. Let's make them part of the module API.

Part of #7228

@TarantoolBot document
Title: Lua/C functions for decimals in the module API

The following functions are added into the module API:

```c
/**
 * Allocate a new decimal on the Lua stack and return
 * a pointer to it.
 */
API_EXPORT box_decimal_t *
luaT_newdecimal(struct lua_State *L);

/**
 * Allocate a new decimal on the Lua stack with copy of given
 * decimal and return a pointer to it.
 */
API_EXPORT box_decimal_t *
luaT_pushdecimal(struct lua_State *L, const box_decimal_t *dec);

/**
 * Check whether a value on the Lua stack is a decimal.
 *
 * Returns a pointer to the decimal on a successful check,
 * NULL otherwise.
 */
API_EXPORT box_decimal_t *
luaT_isdecimal(struct lua_State *L, int index);
```