IsListType was causing ~100 allocations for a non list object. It is
used in a wide range of code, so perform a more targeted check.

The use of `%#v` in a hot return path for `fmt.Errorf()` was the main
victim.

Replace `%#v` with a typed error and create a cache of types that are
lists with a bounded size (probably not necessary, but safer).

```
BenchmarkGet-12          	  100000	    119635 ns/op	   20110 B/op	     206 allocs/op
BenchmarkWatchHTTP-12    	  100000	     65761 ns/op	    7296 B/op	     139 allocs/op

BenchmarkGet-12          	  100000	    109085 ns/op	   17831 B/op	     152 allocs/op
BenchmarkWatchHTTP-12    	  200000	     33966 ns/op	    1913 B/op	      30 allocs/op
```