Introducing feature 'alloc' that removes dependency of alloc::sync::Arc

[jakobdalsgaard]

The naming might be off here -- didn't really know a better feature name.

I'm using Maud for esp no_std development, and the esp-alloc crate I use there, does not have sync::Arc -- so I made a feature in Maud called 'alloc' (perhaps it should be called 'esp-alloc'?) that when enabled, will have Maud skip support for sync::Arc. Works great so far :-)