Add label $name | EXP; fix break

This is to fix the problem where `break` is dynamic, not lexical.

With this it should be possible to do this sort of thing:

    label $break | inputs | if ... then $break|error else . end

This is a backwards-incompatible change for master, but the previous
`break` hadn't shipped yet.

Still needed:

 - testing

tests/all.test

@@ -247,15 +247,35 @@ null
 1
 [1,2,4,8,16,32,64]
 
-[while(.<100; .*2|if . > 10 then break else . end)]
+[label | while(.<100; .*2|if . > 10 then break else . end)]
 1
 [1,2,4,8]
 
+[(label $here | .[] | if .>1 then break $here else . end), "hi!"]
+[0,1,2]
+[0,1,"hi!"]
+
+[(label $here | .[] | if .>1 then break $here else . end), "hi!"]
+[0,2,1]
+[0,"hi!"]
+
+(label | (label | 2 | break2)), 1
+null
+1
+
+%%FAIL
+break
+jq: error: break used outside labeled control structure
+
+%%FAIL
+. as $foo | break $foo
+jq: error: *label-foo/0 is not defined
+
 [.[]|[.,1]|until(.[0] < 1; [.[0] - 1, .[1] * .[0]])|.[1]]
 [1,2,3,4,5]
 [1,2,6,24,120]
 
-[foreach .[] as $item ([3, null]; if .[0] < 1 then break else [.[0] -1, $item] end; .[1])]
+[label | foreach .[] as $item ([3, null]; if .[0] < 1 then break else [.[0] -1, $item] end; .[1])]
 [11,22,33,44,55,66,77,88,99]
 [11,22,33]