ecmult: Use size_t for array indices #1794

I don't think the current code is incorrect, but using size_t improves readability because the type makes it clear that we're dealing with array indices.

Also, making the result of the ECMULT_TABLE_SIZE macro (hopefully) a size_t fixes a compiler warning on MSVC, see #1791.

Concept ACK.

I came to the same idea while working on #1791, but left it for later :)

ACK 9e7aede41eaf5dd6f4da7e233bc90982476f325e, I have reviewed the code and it looks OK.

However, there are still more cases to address:

• size_t i in: https://github.com/bitcoin-core/secp256k1/blob/2d9137ce9d7e576bfada2859292ecaa0887a6b8c/src/ecmult_impl.h#L339-L348
• size_t m in: https://github.com/bitcoin-core/secp256k1/blob/2d9137ce9d7e576bfada2859292ecaa0887a6b8c/src/ecmult_const_impl.h#L93-L98
• size_t i in: https://github.com/bitcoin-core/secp256k1/blob/2d9137ce9d7e576bfada2859292ecaa0887a6b8c/src/ecmult_const_impl.h#L228-L230

Also, making the result of the ECMULT_TABLE_SIZE macro (hopefully) a size_t fixes a compiler warning on MSVC, see #1791.

It does.

• size_t i in: https://github.com/bitcoin-core/secp256k1/blob/2d9137ce9d7e576bfada2859292ecaa0887a6b8c/src/ecmult_impl.h#L339-L348

I think there's no perfect solution here, but fixing this one will open up the entire business of cleaning up types in secp256k1_ecmult_wnaf (see #1795), and I think this should be a separate PR.

The same is true for the stuff in ecmult_gen; I think this could be a separate PR.

re-ACK 47eb70959a91bbfa10dc209e8ccbe7350f5b9b04.

ACK 47eb70959a91bbfa10dc209e8ccbe7350f5b9b04

@theStack Want to review this?

ACK 47eb70959a91bbfa10dc209e8ccbe7350f5b9b04