Move towards using mozilla rust-code-analysis

[mpadge]

https://github.com/mozilla/rust-code-analysis, which would offer the two important advantages of:

1. Allowing tree-sitter to be used for everything, instead of ctags/gtags
2. Allow direct extension to every language supported by rust-code-analysis

The current metrics are documented in https://mozilla.github.io/rust-code-analysis/metrics.html. Although more restricted than the larger number of metrics currently collected here, they also include outputs of several tools that extend analysis beyond current state here. A first step towards this would likely be adding R as a language to rust-code-analysis.