Mock sdk/syscalls for testing

[Stebalien]

We should have a "mock sdk" for native unit testing (i.e., without a wasm VM). For completeness, we should provide both an SDK mock and a low-level syscall mock.

The core design is to introduce:

1. A compile time flag to enable the mock sdk/runtime.
2. A special MockRuntime trait that:
   1. Is invoked by the mock syscalls/sdk.
   2. Is implemented by the user and/or testing framework.
3. A thread-local variable to store a MockRuntime. A unit test would bind a new runtime to said thread-local variable while executing the test.
4. Mock implementations of the syscalls and/or sdk methods that call the MockRuntime currently stashed in the thread-local variable instead of making "real" system calls.

Mock SDK

The first option is to mock the SDK itself. I.e., the high-level methods but not fvm_sdk::sys::*.

Advantages:

• Higher-level and easier to mock.

Disadvantages:

• Can't be used to test the SDK.
• Not appropriate for testing alternative SDKs.

Mock Syscalls

The second option is to mock fvm_sdk::sys. Basically, we'd replace the current FFI declarations with actual functions that call into a mock runtime.

Advantages:

• Low-level and suitable for testing the SDK itself.

Disadvantages:

• Requires re-implementing the FVM's logic around IPLD block IDs.
• Annoying to work with low-level/FFI-safe types (lots of encoding, decoding, raw pointers, etc.