Format is:

    [len][name0][len][value0][len][name1][len][value1]...

The lengths are varint encoded so usually a single byte.

The previous `[]string` had 24 bytes of overhead for the slice and 16
for each label name and value; this one has 16 bytes overhead plus 1
for each name and value.

In `SimpleBuilder.Overwrite` and `Labels.Hash` we use an unsafe
conversion from string to byte slice. `Overwrite` is explicitly unsafe,
but for `Hash` this is a pure performance hack.

Signed-off-by: Bryan Boreham <[email protected]>

Signed-off-by: Julien Pivotto <[email protected]>