Heapster: Avoid shift/reduce conflicts involving bitvector negation

We now ensure that `-` has higher precedence than any other operator using
`happy`'s `%left` pragma. This ensures that `- 42 + 27` parses as `(- 42) +
27`, not `- (42 + 27)` like before.

While I was in town, I added `%expect 0` at the top of the parser to ensure
that `happy` will error out in the future if additional shift/reduce conflicts
are introduced.

Fixes #1794.

heapster-saw/src/Verifier/SAW/Heapster/Parser.y

@@ -22,6 +22,8 @@ import Verifier.SAW.Heapster.UntypedAST
 
 }
 
+%expect 0 -- shift/reduce conflicts
+
 %tokentype      { Located Token                         }
 %token
 '('             { Located $$ TOpenParen                 }
@@ -108,6 +110,7 @@ NAT             { (traverse tokenNat   -> Just $$)      }
 %left     '+'
 %left     '*'
 %nonassoc '@'
+%left NEGPREC
 
 %%
 
@@ -136,7 +139,9 @@ expr ::                                         { AstExpr }
   | NAT                                         { ExNat (pos $1) (locThing $1) }
   | 'unit'                                      { ExUnit (pos $1) }
   | expr '+' expr                               { ExAdd (pos $2) $1 $3 }
-  | '-' expr                                    { ExNeg (pos $1) $2 }
+    -- NB: Give negation the highest possible precedence to avoid shift/reduce
+    -- conflicts with other operators, such as + and *.
+  | '-' expr %prec NEGPREC                      { ExNeg (pos $1) $2 }
   | expr '*' expr                               { ExMul (pos $2) $1 $3 }
   | 'struct' '(' list(expr) ')'                 { ExStruct (pos $1) $3 }
   | lifetime 'array' '(' expr ',' expr ',' '<' expr ',' '*' expr ',' expr ')'