feat: structure as cargo workspace

[chadoh]

Fixes stellar#328

Undoes stellar#232

Forked from stellar#330

Add top-level Cargo.toml, with all project folders as part of this
workspace.

We now encourage everyone to use Cargo workspaces when
building Soroban projects. Let's make it easy to copy-paste these
examples into their projects, rather than requiring special tooling
(like the old stellar contract init behavior) to update the contract's
Cargo.toml.

Currently, there are two compilation errors when attempting to run
tests:

error[E0433]: failed to resolve: use of undeclared crate or module `__get_allowance`
  --> token/src/contract.rs:67:12
   |
67 |     pub fn get_allowance(e: Env, from: Address, spender: Address) -> Option<AllowanceValue> {
   |            ^^^^^^^^^^^^^ use of undeclared crate or module `__get_allowance`
   |
help: there is a crate or module with a similar name
   |
67 |     pub fn __allowance(e: Env, from: Address, spender: Address) -> Option<AllowanceValue> {
   |            ~~~~~~~~~~~

error[E0223]: ambiguous associated type
  --> bls_signature/src/lib.rs:78:18
   |
78 |         agg_sig: Self::Signature,
   |                  ^^^^^^^^^^^^^^^ help: use fully-qualified syntax: `<IncrementContractArgs as soroban_sdk::testutils::ed25519::Sign>::Signature`

What

[TODO: Short statement about what is changing.]

Why

[TODO: Why this change is being made. Include any context required to understand the why.]

Known limitations

[TODO or N/A]

[chadoh]

Closed in favor of stellar#328