This adds the elm compiler, version 0.19.0. This provides the
`elm` command, with the exception of the `elm reactor` subcommand.
Named `elm-compiler`, to leave space for `elm` as the full elm
including reactor.
* gnu/packages/elm.scm: New module.
(elm-compiler): New package.
* gnu/packages/patches/elm-disable-reactor.patch: New patch.
* gnu/packages/patches/elm-fix-map-key.patch: New patch.
* gnu/packages/patches/elm-relax-glsl-bound.patch: New patch.
* gnu/local.mk: Add new files.
Co-authored-by: Ludovic Courtès <ludo@gnu.org>
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