Improved Encoder API

Thanks to @Barinzaya for submitting #44 which improves the encoder API in two ways:

1. It defines a new Output trait and a new encode_into function that allows to encode OSC data into anything that implements the Output trait. The Output trait got implemented of Vec<u8>, so that the existing encode function can leverage encode_into under the hood.
2. Encoder performance improved by an order of magnitude 🚀 🐎 ! There was no regression, only speedups between 5 and 21 times. This was mainly achieved by preventing a lot of allocations (as done in the previous implementations). For more details see #44.