Use direct call to external tool

rfcs/direct_call_to_external_tool.md

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+# Direct call to external toolchain
+
+The compiler shouldn't require a shell in order to work.
+
+# Motivation for the change
+
+I try to fix this https://github.com/esy/esy/issues/1344
+I can reduce process in esy but half of this has to be fixed in ocaml/flexdll
+
+# Technical details of the change
+
+Replace Sys.command by Unix.create_process or Unix.open_process_args
+
+# Drawbacks of the change and alternatives to the change
+
+pros:
+- You earn some time without launching useless shell
+- You avoid the [quoting nightmare](https://github.com/ocaml/ocaml/pull/10727) on OS with exec*
+- You handle better all path (I have not tested path with $,; and other niceties)
+- You workaround cmd limitation of 8187
+
+cons:
+- The flags -pp/-ppx 'binary -args' will not work
+  - User can still create a wrapper script
+  - We can add --ppopt, -ppxopt like -ccopt
+  - We can add -direct_pp,-direct_ppx so we don't break any projects. All good citizens that use dune will get a boost benefit freely.
+- You loose usage of %COMSPEC% on windows (but is it still needed ?)
+  - We can still read that variable and act in consequence
+- We break user that rely on this implementation detail and pass multiple opt to -ccopt/-cclib instead of repeating it.
+  - Workaround either split on space and/or implement word splitting that respect quoting.
+
+cons without workaround:
+- Unix will not be optional anymore (or maybe at least the creation part)
+
+# Unresolved questions