Allow for use of data structures without std by adding alloc feature

[iriswebb]

Most data structures (String, Vec, etc) don't require the standard library to work, since they are also part of the alloc crate. Which can work in no_std environments independent of alloc as long as a #[global_allocator] is provided by the user.

This patch adds an alloc feature and turns most invocations of std data structures into invocations of alloc