Excellent docs on numeric casting in C++

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

まず issue #37 と、そこで参照されているディスカッションおよび pull requests を確認し、意図された警告設定を理解します。次に、リポジトリのビルド設定を調べ、提案された変換警告を有効にしてコンパイルします。警告が有効になり、その結果生じる既存の切り捨て、縮小変換、オーバーフロー、または暗黙の変換に関する警告が修正されるか、明示的に抑制されれば完了です。