tpl: Allow 'Querify' to take lone slice/interface argument

Querify can now take a lone string/interface slice (with string
keys) as a parameter, or multiple string parameters, to build
URL queries.

Querify earlier used 'Dictionary' to add key/value pairs to a
map to build URL queries. Changed to dynamically generate ordered
key/value pairs. Cannot take string slice as key (earlier
possible due to Dictionary).

Added tests and benchmarks for querify.

Closes #6735

tpl/collections/init.go

@@ -122,6 +122,10 @@ func init() {
 					`<a href="https://www.google.com?{{ (querify "q" "test" "page" 3) | safeURL }}">Search</a>`,
 					`<a href="https://www.google.com?page=3&amp;q=test">Search</a>`,
 				},
+				{
+					`{{ slice "foo" 1 "bar" 2 | querify | safeHTML }}`,
+					`bar=2&foo=1`,
+				},
 			},
 		)