Add date builtins (fix #364)

Windows support for strptime() is missing.  We can add a copy of one
from a BSD later.

tests/all.test

@@ -1140,6 +1140,10 @@ bsearch(4)
 [1,2,3]
 -4
 
+[strptime("%Y-%m-%d %H:%M:%SZ")|(.,mktime)]
+"2015-03-05 23:51:47Z"
+[[2015,2,5,23,51,47,4,63],1425621107]
+
 # module system
 import "a" as foo; import "b" as bar; def fooa: foo::a; [fooa, bar::a, bar::b, foo::a]
 null