Pre-process extracted examples before running RMC on them.

[bdalrhm]

Before we run RMC on the extracted examples from the Rust books, we need to do the following pre-processing steps:

• Extract compiler annotations in code blocks and translate them to appropriate RMC flags.
• Look for custom configs in cases where the extracted annotations are not enough (e.g., unwind flag and codegen fail step). #407
• Look for custom assertions to add to the examples. #408

[adpaco-aws]

Closing this issue because the tools to do this are already there.

We only need to study the examples and include assertions on a case-by-case basis. Opened #517 for that.