Add reproc::fill for sending stdin after process starts

[stingray-11]

Taking a second attempt at this. See issue #67. My program uses reproc to drain/fill with ~50MB in/out per process.

This time I implemented fill() in nearly the same way as the existing drain(), including a callback template and a default for strings. Also, I implemented the analogous code in the C portion the same as drain(). Also added examples and tested. The drain-fill tests now compare the input/output to actually check both drain/fill are working properly.

Also included are a couple minor fixes - use SYSTEM in cmake so reproc warnings don't show up in users of the library, and add the ability to specify a custom buffer size for fill/drain (64K rather than 4K reduces my program's CPU usage by ~20%).

[stingray-11]

Some tests are failing but not because of these changes... macOS is missing clang-tidy and windows doesn't like a #define in reproc.c.