stmt: Support iterating over vector values

I ran into wanting to iterate over just the values of a vector and wondering
whether that could just work.

This adds support for the following, where v will be value of vec[i].

    local vec = vector("zero", "one", "two");

    for ( i, v in vec )
        print i, v;

testing/btest/Baseline/language.vector-for-value/out

@@ -0,0 +1,13 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+0, zero
+1, one
+2, two
+4, four
+0, zero
+1, one
+2, two
+4, four
+zero
+one
+two
+four