Support toSignedInteger in reference evaluator

Checks off one box in #1249.

CHANGES.md

@@ -15,6 +15,9 @@
   were incorrectly computed.
   ([#1773](https://github.com/GaloisInc/cryptol/issues/1773))
 
+* The reference evaluator now evaluates the `toSignedInteger` primitive instead
+  of panicking.
+
 ## New features
 
 * REPL command `:dumptests <FILE> <EXPR>` updated to write to stdout when

src/Cryptol/Eval/Reference.lhs

@@ -758,6 +758,10 @@ by corresponding type classes:
 >   , "lg2"        ~> vFinPoly $ \n -> pure $
 >                     VFun $ \v ->
 >                       vWord n <$> appOp1 lg2Wrap (fromVWord =<< v)
+>   , "toSignedInteger"
+>                  ~> vFinPoly $ \_n -> pure $
+>                     VFun $ \x ->
+>                     VInteger <$> (fromSignedVWord =<< x)
 >   -- Rational
 >   , "ratio"      ~> VFun $ \l -> pure $
 >                     VFun $ \r ->

tests/issues/issue1249.icry

@@ -0,0 +1,12 @@
+// toSignedInteger
+
+toSignedInteger (0 : [64])
+:eval toSignedInteger (0 : [64])
+toSignedInteger (1 : [64])
+:eval toSignedInteger (1 : [64])
+toSignedInteger (2^^63 - 1 : [64])
+:eval toSignedInteger (2^^63 - 1 : [64])
+toSignedInteger (-1 : [64])
+:eval toSignedInteger (-1 : [64])
+toSignedInteger (-(2^^63) : [64])
+:eval toSignedInteger (-(2^^63) : [64])

tests/issues/issue1249.icry.stdout

@@ -0,0 +1,11 @@
+Loading module Cryptol
+0
+0
+1
+1
+9223372036854775807
+9223372036854775807
+-1
+-1
+-9223372036854775808
+-9223372036854775808