Rust bindings are incompatible with EIP-2003

[axic]

EIP-2003 states that code and (host) context is not mandatory for precompiles and can be null, but the Rust bindings assert for the presence of both.

[chfast]

For code, we don't disallow null pointers. We can clarify that the pointer cannot be if the size is non-zero. You will be always allowed to pass empty code.

Context is actually two things:

• the pointer to additional data Host wants to keep with given execution (the real context), this can always be empty,
• the Host interface (vtable with methods).
  These two are going to be split in the next version of EVMC, but it still do not solve the problem entirely.

Anyway, I think this is fine for now while we don't have any EVMC precompiles.

[axic]

It should be clarified in evmc.h because it doesn't say anything about these fields.