Measure trait and dynamic buffers

[jac3km4]

Another PR with some of my changes.
This aims to make the TryWrite trait more convenient to use by adding an alloc feature, that enables the user to convert values directly into bytes.
The way this is achieved is by adding a new Measure trait that can calculate the required size of the buffer for a value. This is used in a new IntoBytesExt extension trait, which is feature-gated behind alloc - it allocates a new Vec and writes the value into it.