Introduce enum evmc_bool

[axic]

In https://github.com/ethereum/evmc/blob/master/include/evmc/evmc.h#L164 evmc_block_hash_fn returns 1 on success, 0 on failure.

Compared to that, evmc_status_code uses the POSIX style 0 for success, anything else for failure.

This kind of indicates an inconsistency. evmc_block_hash_fn tries to map a boolean into a uint, that's why.

A solution could be either using stdbool.h or creating an enum to replicate it: evmc_bool.

[chfast]

I considered this and I'm not convinced because I'm not sure we should introduce another bool type.

[axic]

stdbool.h: http://pubs.opengroup.org/onlinepubs/009604499/basedefs/stdbool.h.html, not sure which C standard introduced it.

[axic]

Should check stdbool.h with Go and rust-bindgen.

[chfast]

The stdbool.h is good for:

• Visual Studio
• Go
• Python CFFI
• Rust.

I will try this one.