feat(decimal): don't use tarantool symbols for decimal support

Before this all decimal operations were implemented using the tarantool decimal api, which means that even the simplest of operations like creating a zero number or comparing to numbers would require a call to a function in the tarantool runtime. This forced some of our users to link against tarantool just for the decimal support.

Now decimals are implemented using the dec crate. It is backed by the same decNumber library as the tarantool's decimal numbers. And it is statically liked at build time, so the code that only uses decimals can be tested without the need for tarantool runtime.