FFI: Add a stdbool type for C99 _Bool/C23 bool

[williewillus]

I misread the FFI manual, and mistakenly used the FFI boolean type to represent a C99 _Bool.

Since Chez Scheme predates C99 being common, boolean in the FFI actually just means int, which was what was commonly used before C99 _Bool. This means I was reading 3 bytes of garbage every time a C function returned a bool, causing things to evaluate to #t on the scheme side when they shouldn't.

Now that it's 2024 and C99 _Bool is pretty widely used, and C23 is even finally reserving the keyword bool for the type, so I think the FFI should add stdbool to explicitly represent this type on the Chez side. Currently, I have to assume that sizeof(bool)==1 on my platform, use unsigned-8, and do the zero-check myself. Although it's rare that someone would make their platform bools larger than 1 byte, hardcoding the size and having to do the check myself is still a minor annoyance.