xml: rewrite the xml generator to avoid xml.Encoder

This commit intoduces a mostly hand-rolled XML encoder, inspired by xml.Encoder. Since plist is a simple format that doesn't require attributes, only has one data type with input in need of encoding, and is otherwise constrained, we don't need the full power of encoding/xml.

As a side effect, we can now elide the contents of empty elements (<key></key> -> <key/>).

The speed, memory requirements and number of allocations are all improved.

Fixes https://github.com/DHowett/go-plist/issues/34.

name                    old time/op    new time/op    delta
XMLEncode-4               25.9µs ± 3%    14.9µs ± 1%   -42.66%
BigXMLGenerate-4          47.3ms ± 1%    23.7ms ± 1%   -49.91%
BigPrettyXMLGenerate-4    55.0ms ± 1%    30.0ms ± 1%   -45.42%
XMLGenerate-4             16.0µs ± 1%     5.8µs ± 1%   -63.81%

name                    old alloc/op   new alloc/op   delta
XMLEncode-4               8.71kB ± 0%    6.42kB ± 0%   -26.26%
BigXMLGenerate-4          2.62MB ± 0%    1.60MB ± 0%   -39.02%
BigPrettyXMLGenerate-4    2.62MB ± 0%    1.60MB ± 0%   -39.02%
XMLGenerate-4             5.83kB ± 0%    4.50kB ± 0%   -22.77%

name                    old allocs/op  new allocs/op  delta
XMLEncode-4                 88.0 ± 0%      63.0 ± 0%   -28.41%
BigXMLGenerate-4           86.7k ± 0%     72.6k ± 0%   -16.24%
BigPrettyXMLGenerate-4     86.7k ± 0%     72.6k ± 0%   -16.24%
XMLGenerate-4               54.0 ± 0%      31.0 ± 0%   -42.59%