Add Alternate Mocking Framework (1-of-3 / Initial Scaffolding)

[sappenin]

High Level Overview of Change

• Add mockall mocking framework.
• Introduce a new trait that defineds all host functions (host_bindings_trait.rs)
• Wire-in proper host function definitions for different build profiles.
• Add sample test in error_code.rs
• Update host function audit script to use correct repo and check all host function definitions in each profile.

Context of Change

This PR is number-1 in a series of three PRs that offers an alternative design to the proposal in #102. In particular:

• PR #117 (this PR): Adds initial mocking support using the mockall framework.
• PR #118: Updates existing unit tests to use the new mocking framework, where appropriate.
• PR #119: Adds new unit test coverage to a variety of host-function usages in the library.

Note on LOC in this PR

The astute observer will notice that this PR has a reasonably large number of lines-of-code changed. There are two primary culprits here, mostly falling into the changes required for host_bindings_wasm.rs. Overall, to pull off our current design (where all host functions are accessible from the host:: namespace), we need to replicate the host functions (there are quite a lot of them on their own) three times. The first is to define the C abstraction (required by rippled/WASM VM). The second is to implement the trait, which delegates to the C abstraction (trait is required for mocking), and the third is to hide the overall implementation of the first two (to maintain the host:: namespace).

While this is a large changeset, the changes are mostly just boilerplate. I've tried to reduce this boilerplate as much as possible using macros. This helped, but not everything can be macro'd.