Excellent docs on numeric casting in C++

還沒有人認領這個 Issue。

Without strict compiler warnings, it is possible to write quite a bit of C++ without it be obvious once needs to study the details of numeric casting. Implicit conversion will convert various integer types to other integer types and to floating point types, quietly and hopefully correctly.

Because we can't have correct code based on hope, we should instead enable warnings from the get go that warn about implicit conversion and specifically truncating or narrowing conversion and convertions that might overflow.

So, we want to enable warnings like `-Wconversion` per #37.

What this means is that you'll get loud warnings right away about converting types and will need to figure out how to avoid or suppress them correctly, to fix the potential existing bugs they hint at and avoid creating new bugs in the process.

Refs https://github.com/mapbox/node-cpp-skel/pull/78#discussion_r144410926 https://github.com/mapbox/carmen-cache/issues/94
refs https://github.com/mapbox/gzip-hpp/pull/19#issuecomment-336537155

/cc @mapsam @GretaCB @flippmoke

先檢視 issue #37 以及其中參照的討論和 pull requests，以了解預期的警告設定。接著檢查儲存庫的建置設定，並使用提議的轉換警告進行編譯。當警告已啟用，且由此產生的現有截斷、窄化、溢位或隱式轉換警告已獲得處理或明確抑制時，即表示完成。

由索引模型根據 Issue 內容生成。