Release 0.18.0-rc

[fintelia]

One open question is whether we should take this chance to remove Decoder::{next_row, next_interlaced_row} in favor of Decoder::read_row (added in #493)

[anforowicz]

My thoughts on removing Decoder::{next_row, next_interlaced_row}:

• I think this shouldn't block the 0.18 release
• If we want to remove those APIs, then I think the first step would be to mark them as #[deprecated]. I think it doesn't matter that much if we do it in 0.18.0 vs 0.18.1 or later. And then we can remove them in 0.19.
• I don't have a strong opinion about desirability of removing those APIs:
  • The old APIs are sometimes convenient (I guess mostly because buffer management on Rust side is easier than buffer management on C++ side). Current Chromium implementation uses those old APIs (see here; I now realize that the TODO/bug link should be updated), although using the new API would force an improvement for some scenarios (avoiding an extra memcpy here)
  • On the other hand, the old APIs are kind of redundant (i.e. removing them will result in smaller, simpler-to-understand API)