This PR adds support for IOs with custom encodings on Windows, using https://github.com/pffang/libiconv-for-Windows, which wraps GNU libiconv in an MSVC project.

To make things work more generically across platforms, there is now a new flag -Dwithout_iconv which disables both libiconv and the built-in iconv_* functions from the C library, plus all the associated tests; any attempt to use custom encodings will raise during runtime, similar to Windows before this PR. This is useful when the target platform has a poor iconv implementation (e.g. Android NDK) and libiconv is not yet available on that platform. The code below is all that is needed to select between libiconv and iconv at the moment:

# src/crystal/iconv.cr
{% if flag?(:win32) %}
  require "./lib_iconv"
  private USE_LIBICONV = true
{% else %}
  require "c/iconv"
  private USE_LIBICONV = false
{% end %}

Thus Windows is hardcoded to use libiconv, and other platforms will continue to use iconv, but it is easy to change this in the future. If -Dwithout_iconv is provided then src/crystal/iconv.cr itself would not be required.

The decision to distinguish LibIconv from LibC is intentional, because they are not strictly API-compatible (e.g. FreeBSD has __iconv, GNU libiconv has iconvctl).

Related: #5430