- moved workspace methods to the workspace class

- replaced dict with array

- added tests for build output parsing functions
- fixed year in headers
- fixed parsing of `rustc-flags` ([can be a set of flags](https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script))

- member names and paths are automatically resolved by interpreting the workspace manifest
- the list of member names in the BUILD file is no longer necessary

- binary and library targets aren’t enabled yet
- refactored target class hierarchy

- the toolchain can be defined in `pants.ini`

Example

```
[bootstrap.cargo]
toolchain: nightly-2018-12-31
```

- the value can be a name or a path to a rust-toolchain file
- the default value is `nightly-2018-12-31`
- removed the toolchain path in the cargo_workspace target

- added examples for binary, library and workspace targets

- removed duplicate log message

- switched default toolchain to explicit toolchain to have the highest precedence
- CI tests failed because the `rust-toolchain` file overwrote the default toolchain
- [More information about the toolchain override precedence](https://github.com/rust-lang/rustup.rs#override-precedence)

- made the fingerprint of the rustup install script configurable via  the option `--script_fingerprint`
- the rustup install script is only downloaded once and stored in `<versioned_workdir>/rustup_install_script/rustup.sh`
- the task tries to find the executable of rustup in the default location `~/.cargo/bin` if the task can’t find the executable via the `PATH` variable

- fixed AttributeError for self.LAST_KNOWN_FINGERPRINT

To build always valid binaries, the compile task implements a simple solution to support `cargo:rerun-if-env-changed` and `cargo:rerun-if-changed` statements.
If a build script target contains a `cargo:rerun-if-env-changed` or `cargo:rerun-if-changed` statement, the compile task marks that target and its dependent targets as invalid. The task itself doesn’t actually check if an environment variable or a file has changed.

- to have no duplicate paths in make_dirs and make_sym_links

- Cancel the execution of pants if the execution of a subprocess call in the tasks `fetch`, `build` or `bootstrap` fails.
- Show `std_out` and `std_err` if the execution of a build script fails.

- fixed a bug in program_rule (multiple targets may depend on a build script)

- libraries of the kind `rlib`, `dylib`, and `staticlib` are now supported
- removed the `CargoSyntheticProcMacro` target
- cargo libraries of the kind `proc-marco` are now treated as `CargoSyntheticLibrary`
- replaced the workunit outcome with the return code
- removed the logging of the workunit outcome
- added the env variables of a test invocation to the `rust_test` product (if the `DYLD_LIBRARY_PATH` is missing, the proc-macro test will fail)

- added build, fetch integration tests
- added build, workspace tests
- refactored basic, custom build rules to reduce code duplications
- added build_flags fingerprint strategy (additional cargo options such as `release` or `test` are now taken into account in the fingerprint)
- added additional examples
- removed cargo lock files in rust examples
- changed dist path of libraries and binaries from `dist/lib`, `dist/bin` to `dist/rust/lib`, `dist/rust/bin`
- the way in which the fingerprint of a crate is calculated has been changed to prevent crates from being compiled multiple times