Add support for no-alloc using static lifetime ref Box and Vec types

[leighmcculloch]

What

Add some super limited support for type construction in no-alloc envs using static lifetime ref Box and Vec types, used whenever the alloc feature is disabled. The alloc feature is dependent on the alloc crate's Box and Vec, and is enabled whenever std is enabled.

This results in three tiers of capabilities:

• std, type construction using the default allocator for recursive types (that use Box) and Vecs, and encoding/decoding.
• alloc, type construction using the default allocator for recursive types (that use Box) and Vecs.
• Otherwise, type construction using static lifetime refs for recursive types and Vecs.

Why

This crate has good support for no_std environments, but not for environments that require no allocator. That's because the XDR types for the Stellar protocol are recursive, and so we can't easily drop-in stack based Vec types like heapless. There are some uses of this lib in no alloc environments that involve using the types for structured programming without us actually needing encoding/decoding. This PR makes that possible without changing the interface of passing an allocator around. Ideally in #39 this is revisited and an allocator is passed down instead.

Known limitations

These changes are temporary, and will be replaced when #39 is addressed.

[leighmcculloch]

Updates the builds for https://github.com/stellar/terraform/pull/804.