Avoid being a build-dependency

[rasky]

skeptic is a helper for tests, and as such it must and should be a dev-dependency. Unfortunately, the way it plugs into build.rs causes all downstream users of libraries using skeptic to also compile and build skeptic and all its dependencies, for the purpose of generating a test file that will never be executed. The dependency tree is not so small as one would hope:

│   │   [build-dependencies]
│   │   └── skeptic v0.13.4
│   │       ├── bytecount v0.4.0
│   │       ├── cargo_metadata v0.6.4
│   │       │   ├── error-chain v0.12.0
│   │       │   │   └── backtrace v0.3.9
│   │       │   │       ├── cfg-if v0.1.6 (*)
│   │       │   │       ├── libc v0.2.44 (*)
│   │       │   │       └── rustc-demangle v0.1.9
│   │       │   ├── semver v0.9.0
│   │       │   │   ├── semver-parser v0.7.0
│   │       │   │   └── serde v1.0.82
│   │       │   ├── serde v1.0.82 (*)
│   │       │   ├── serde_derive v1.0.80
│   │       │   │   ├── proc-macro2 v0.4.24 (*)
│   │       │   │   ├── quote v0.6.10 (*)
│   │       │   │   └── syn v0.15.22
│   │       │   │       ├── proc-macro2 v0.4.24 (*)
│   │       │   │       ├── quote v0.6.10 (*)
│   │       │   │       └── unicode-xid v0.1.0 (*)
│   │       │   └── serde_json v1.0.33
│   │       │       ├── itoa v0.4.3
│   │       │       ├── ryu v0.2.7
│   │       │       └── serde v1.0.82 (*)
│   │       ├── error-chain v0.12.0 (*)
│   │       ├── glob v0.2.11
│   │       ├── pulldown-cmark v0.2.0
│   │       │   └── bitflags v1.0.4 (*)
│   │       ├── serde_json v1.0.33 (*)
│   │       ├── tempdir v0.3.7
│   │       │   ├── rand v0.4.3
│   │       │   │   └── libc v0.2.44 (*)
│   │       │   └── remove_dir_all v0.5.1
│   │       └── walkdir v2.2.7
│   │           └── same-file v1.0.4

Given that rust is not... fast at builds, I think that adding all this overhead is a disservice for users of libraries using skeptic.

Is it possible to devise a better way to handle this?

[rasky]

My own two cents: create a skeptic binary and let users of skeptic install it through cargo install. Running skeptic README.md generates the test file. The test file itself contains a checksum for README.md and checks it at every run; if it changes, it suggests the developer to re-run skeptic README.md (or re-run it itself?).

[rasky]

Duplicate of #60