Adds an alternative TOML configuration format to Lake.
* Uses TOML v1.0.0 and is fully specification compliant (tested via
[toml-test v1.4.0](https://github.com/toml-lang/toml-test/tree/v1.4.0)).
* Supports package configuration options, Lean libraries, Lean
executables, and dependencies.
* TOML configurations can be generated for new projects via `lake
new|init <pkg> <template>.toml`.
* Supported configurations can be converted to/from TOML via `lake
translate-config <lang>`.
This adds a `test_extern` command.
Usage:
```
import Lean.Util.TestExtern
test_extern Nat.add 17 37
```
This:
* Checks that the head symbol has an `@[extern]` attribute.
* Writes down `t == t'`, where `t` is the term provided, and `t'` is the
reference implementation (specifically, `t` with the head symbol
unfolded).
* Tries to reduce this to `true`, and complains if this fails.
Note that the type of the term must have a `BEq` instance for this to
work: there's a self-explanatory error message if it isn't available.
