Remove dependency on Janestreet Core

Core doubles the closure size and adds 8mb to the binary size for dubious benefit.
This adds FileUtils to do the file interaction bits that aren't in the
stdlib and removes Core in preference to the bundled Stdlib.

Tests are passing, but I want to investigate the nix build before I
commit to this approach

lib/dune

@@ -1,15 +1,8 @@
 (library
  (name lib)
  (public_name flake_env.lib)
- (libraries
-  core
-  core_unix
-  core_unix.filename_unix
-  core_unix.sys_unix
-  ppx_yojson_conv
-  re
-  sha)
+ (libraries fileutils ppx_yojson_conv re sha)
  (instrumentation
   (backend bisect_ppx))
  (preprocess
-  (pps ppx_yojson_conv ppx_jane)))
+  (pps ppx_yojson_conv)))