allow specification of languages by name; add decompilers to languages

allow specification of languages by name; add decompilers to languages

* module/system/base/language.scm (lookup-decompilation-order): New
  function, like its compiling cousin, but backwards.
  (compute-translation-order): Rework so that languages can be specified
  either by name or by identity. Return a list of language - procedure
  pairs, without the "to" language in the list, instead of a list of
  languages.
  (invalidate-compilation-cache!): Invalidate the decompilation cache
  too.
  (<language>): Add a decompiler field.

* module/system/base/compile.scm (compile-passes): Much simpler now that
  lookup-compilation-order gives us the procedures directly.

* module/language/*/spec.scm: Specify compilers by name, so that we can
  avoid unnecessary module loads, and so that when we specify
  decompilers, we can avoid cycles.