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.

module/language/assembly/spec.scm

@@ -21,7 +21,6 @@
 
 (define-module (language assembly spec)
   #:use-module (system base language)
-  #:use-module (language bytecode spec)
   #:use-module (language assembly compile-bytecode)
   #:export (assembly))
 
@@ -31,5 +30,5 @@
   #:reader	read
   #:printer	write
   #:parser      read ;; fixme: make a verifier?
-  #:compilers   `((,bytecode . ,compile-bytecode))
+  #:compilers   `((bytecode . ,compile-bytecode))
   )

module/language/bytecode/spec.scm

@@ -21,7 +21,6 @@
 
 (define-module (language bytecode spec)
   #:use-module (system base language)
-  #:use-module (language objcode spec)
   #:use-module (system vm objcode)
   #:export (bytecode))
 
@@ -33,5 +32,5 @@
   #:version	"0.3"
   #:reader	read
   #:printer	write
-  #:compilers   `((,objcode . ,compile-objcode))
+  #:compilers   `((objcode . ,compile-objcode))
   )

module/language/ghil/spec.scm

@@ -21,7 +21,6 @@
 
 (define-module (language ghil spec)
   #:use-module (system base language)
-  #:use-module (language glil spec)
   #:use-module (language glil)
   #:use-module (language ghil)
   #:use-module (language ghil compile-glil)
@@ -41,5 +40,5 @@
   #:reader	read
   #:printer	write-ghil
   #:parser      parse
-  #:compilers   `((,glil . ,compile-glil))
+  #:compilers   `((glil . ,compile-glil))
   )

module/language/glil/spec.scm

@@ -21,7 +21,6 @@
 
 (define-module (language glil spec)
   #:use-module (system base language)
-  #:use-module (language assembly spec)
   #:use-module (language glil)
   #:use-module (language glil compile-assembly)
   #:export (glil))
@@ -38,5 +37,5 @@
   #:reader	read
   #:printer	write-glil
   #:parser      parse-glil
-  #:compilers   `((,assembly . ,compile-asm))
+  #:compilers   `((assembly . ,compile-asm))
   )

module/language/objcode/spec.scm

@@ -21,7 +21,6 @@
 
 (define-module (language objcode spec)
   #:use-module (system base language)
-  #:use-module (language value spec)
   #:use-module (system vm objcode)
   #:use-module (system vm program)
   #:export (objcode make-objcode-env))
@@ -49,5 +48,5 @@
   #:version	"0.3"
   #:reader	#f
   #:printer	write-objcode
-  #:compilers   `((,value . ,objcode->value))
+  #:compilers   `((value . ,objcode->value))
   )

module/language/scheme/spec.scm

@@ -22,7 +22,6 @@
 (define-module (language scheme spec)
   #:use-module (system base language)
   #:use-module (language scheme compile-ghil)
-  #:use-module (language ghil spec)
   #:export (scheme))
 
 ;;;
@@ -46,7 +45,7 @@
   #:version	"0.5"
   #:reader	read
   #:read-file	read-file
-  #:compilers   `((,ghil . ,compile-ghil))
+  #:compilers   `((ghil . ,compile-ghil))
   #:evaluator	(lambda (x module) (primitive-eval x))
   #:printer	write
   )