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/.gitignore

@@ -1,6 +1,7 @@
 /*/*
 
 # The above pattern doesn't match submodules
+/asio
 /bitsery
 /function2
 /ghc_filesystem
@@ -8,6 +9,7 @@
 /vst3
 
 # And we obviously don't want to ignore our overlays
+!/packagefiles/asio
 !/packagefiles/bitsery
 !/packagefiles/function2
 !/packagefiles/ghc_filesystem