exact-int 24-bit int#47

Some hardware has support for 24-bit ints, such as the eZ80 processor. Some Texas Instruments calculators use this processor (such as the TI-84 Plus CE) but exact-int doesn't give any way to use it's 24-bit ints. Could you consider adding 24-bit int support to exact-int? There's an overview of the CPU [here](https://ce-programming.github.io/toolchain/static/hardware.html#the-24-bit-ez80-cpu), along with the size of its integers. Secondly, (maybe this should be another issue), I think it would make sense to have a "min-int" mode (maybe via a define to enable it, or to have different macros along with the regular ones, eg. prefixed with `m`) where instead of the ints being _exactly_ n-bits, it's only guaranteed to be _at least_ that many bits. Most of the time, I wouldn't care if an int was bigger than I asked, only smaller. Otherwise, if you only need a 24-bit int and want to support both regular CPUs and awkward ones, you would need to check if 24-bit ints are supported, when all you care about is that it is at least that many bits. Eg. When you use a 24-bit int on a normal CPU, it would actually be 32-bits. Thanks