Make first(g) more efficient: extract only 1 value

first(g) was extracting two values, which, if g is slow, made first(g)
slow.  And if the second extraction were to throw an error, then
first(g) would throw that error, which is clearly not the right thing to
do.  Besides that, first(g) was allocating garbage, which it no longer
does.

tests/jq.test

@@ -303,6 +303,11 @@ null
 10
 [0,9,0,5,"nth doesn't support negative indices"]
 
+# Check that first(g) does not extract more than one value from g
+first(1,error("foo"))
+null
+1
+
 #
 # Check that various builtins evalute all arguments where appropriate,
 # doing cartesian products where appropriate.