Add accelerator and fpcore

Author: JonasRegehr

repro.fpcore

@@ -0,0 +1 @@
+(FPCore (z0) (- 1/2 (* (cos (+ z0 z0)) 1/2)))

src/platforms/herbie20.rkt

@@ -199,3 +199,10 @@
 
 (define-operation (binary32->binary64 [x <binary32>]) <binary64>
   #:spec x #:fpcore (! :precision binary64 (cast x)) #:impl identity #:cost 64)
+
+(define-operation (test [z0 <binary64>])
+                  <binary64>
+                  #:spec (- 1/2 (* (cos (+ z0 z0)) 1/2))
+                  #:impl (from-rival)
+                  #:fpcore (! :precision binary64 (test z0))
+                  #:cost 0)