Tool specs and fetch/install/pin refactoring

[mikrostew]

I was planning to only do #173, but some of that was also tangled up with #174, so I combined both into this PR.

Notes

First thing I implemented was tool::ToolSpec, to encapsulate a tool (Node/Yarn/Npm/Npx/Package) and a VersionSpec. This makes it easy to use for fetch/install/pin, which I abstracted to general-purpose functions instead of having things like install_node and install_yarn and install_xxx. That in turn made the command implementation simpler, since they no longer have to parse out the tool and figure out which function to call.

I also initially implemented a tool::Tool enum (like in #173), but didn't need it for anything so I took it back out.

I didn't move the shim code into tool::ToolSpec as mentioned in #173. It didn't seem to fit at this point, but that may need to be part of #175.

Bonus

Fixed a bug in notion-core/src/toolchain/mod.rs where the platform file had been changed to using JSON (in #220), but saving an empty platform instance would result in [platform] being written to the file.

Closes #173
Closes #174
Closes #87

[charlespierce]

Looks great! So much tool-specific code deleted!

[dherman]

Awesome work! I have a few questions about some of the type structure. It might help to chat about some of them -- ping me on discord whenever.

[dherman]

@mikrostew and I spent some time thinking about how to make it more static and it was nonobvious enough that I think it's better to go with the enum for now and file a code quality issue for making it more static later.

[dherman]

Looks great!