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

首先查看 issue #37 以及其中引用的讨论和 pull requests，以了解预期的警告配置。然后检查仓库的构建配置，并使用提议的转换警告进行编译。当警告已启用，并且由此产生的现有截断、窄化、溢出或隐式转换警告已得到处理或被明确抑制时，即可视为完成。