Make the data from an ExpectFile accessible

[quad]

I'm using expect-test in probably a weird semi-abusive way.

I'm writing tests for a network protocol. My tests run in one of three modes:

1. Record: a normal expect-test flow with UPDATE_EXPECT=1; records both the inputs and outputs to the live system
2. Check: a normal expect-test flow with UPDATE_EXPECT=0; asserts on both the inputs and outputs and fails if they've changed
3. Replay: uses .data to supply the inputs and asserts only on the outputs from the implementation.

It would be helpful if ExpectFile let me see into underlying data as easily as Expect does.

[matklad]

This definitely makes sense. Could you bump a version in Cargo.toml to 1.5.0 so that this automatically gets released?

[matklad]

Oh, and mention this in the changelog!