Add fletcher16-gen developer util

1. I'm not convinced our fletcher16 implementation is proper
2. It is not in coreutils

So we add some basic code to do that.

This is also the first dev-util, so add some documentation to DEVEL.md.

DEVEL.md

@@ -49,6 +49,16 @@ Permissable folders:
 - `src`: the upstream unmodified source code. This may be either a submodule or
   nonexistant.
 
+Furthermore, there is a special top-level dir `dev-utils`. In here are
+appropriate utilities that are often useful for development and not generally
+included on normal systems, or are specific to live-bootstrap. Each program
+should be contained within its own directory and include:
+
+- source code
+- compilation script
+
+The directory m2-functions is used for M2-Planet functions (currently).
+
 ## Conventions
 
 - **Patches:**