C API level 0: core execution

[chfast]

This is proof-of-concept of C API level 0 for Ethereum precompiles.

The level 0 expose core execution functionality of a precompile using common function signature.
The function receives input and output byte buffers.
It returns a pair: error code and number of bytes written to the output buffer (output size).

The user of the API must compute the gas cost and the maximum output size on its own.

[chfast]

We are debating how to handle the case where user provides output buffer not big enough to handle the output produced by the library. This situation is programmer error caused by incorrectly computing the max output size and must be fixed in code. Reasonable handling is to abort/panic the process.

1. The output_size parameter may be removed and library should assume the buffer is big enough. Related error can only be detected by crashes or ASan tools. This is also not compatible with Rust from_raw_parts().
2. The library aborts execution with some log.
3. The library reports the special error code which must be handled differently than EVM exceptions. The client does not have much options than abort in this case.