Compile fluid-set! to VM opcode

* libguile/vm-engine.c (fluid-set!): Fix name of opcode to correspond
  with name of Tree-IL primitive.  Fixes compilation of fluid-set! to
  actually use the fluid-set! opcode.
* doc/ref/vm.texi (Dynamic Environment Instructions): Update.
* module/language/cps/compile-bytecode.scm (compile-function): Add
  fluid-set! case.
* module/system/vm/assembler.scm: Update export name for
  emit-fluid-set!.

module/system/vm/assembler.scm

@@ -124,7 +124,7 @@
             emit-pop-fluid
             emit-current-thread
             emit-fluid-ref
-            emit-fluid-set
+            emit-fluid-set!
             emit-string-length
             emit-string-ref
             emit-string->number