feat: structure as cargo workspace

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`
```

Cargo.toml

@@ -0,0 +1,36 @@
+[workspace]
+resolver = "2"
+members = [
+  "./*",
+]
+exclude = [
+  ".git",
+  ".github",
+  ".vscode",
+  "cross_contract",
+  "deployer",
+  "target",
+  "test",
+  "upgradeable_contract",
+  "webauthn",
+  "workspace",
+]
+
+[workspace.dependencies]
+soroban-sdk = "22.0.0-rc.3.2"
+soroban-token-sdk = "22.0.0-rc.3.2"
+
+[profile.release]
+opt-level = "z"
+overflow-checks = true
+debug = 0
+strip = "symbols"
+debug-assertions = false
+panic = "abort"
+codegen-units = 1
+lto = true
+
+# For more information about this profile see https://soroban.stellar.org/docs/basic-tutorials/logging#cargotoml-profile
+[profile.release-with-logs]
+inherits = "release"
+debug-assertions = true

README.md

@@ -13,3 +13,8 @@ to have significant errors and security vulnerabilities. They should not be
 relied on for any purpose.
 
 Join us In the [Stellar Developers Discord server](https://discord.gg/stellardev)
+
+# Build & Run Tests
+
+- `soroban contract build` to generate all `.wasm` files, which are needed for cross-contract dependencies
+- `cargo test`