Replace Boost.Asio with standalone Asio library

We had to add an even hackier hack now to get Boost.Process to
interoperate with Asio's IO contexts. This will be replaced later when
we replace Boost.Process.

subprojects/asio.wrap

@@ -0,0 +1,9 @@
+[wrap-git]
+url = https://github.com/chriskohlhoff/asio.git
+# This is tag asio-1-22-1
+revision = bba12d10501418fd3789ce01c9f86a77d37df7ed
+depth = 1
+patch_directory = asio
+
+[provide]
+asio = asio_dep