mrc-3725: tidy up epic branch

[richfitz]

Misc tweaks to tidy up this branch ahead of merge:

• remove some old unused code (0715222e43a8c175265e4140a5a30b0796cf8e77)
• expose dust versions now that the most recent dust is included (was in #274) (8ad35dbbb3fbcb7d39a0be7bef44fd266da90b3b)
• removed old comments, unused tests, light tidy up in places
• enabled some js tests that now work
• make tests robust to lack of V8

Merges into the epic branch, after which #257 can be reviewed itself

[codecov[bot]]

Codecov Report

Base: 99.52% // Head: 100.00% // Increases project coverage by +0.47% :tada:

Coverage data is based on head (cbee1ab) compared to base (716429f). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## mrc-3182 #275 +/- ## ============================================= + Coverage 99.52% 100.00% +0.47% ============================================= Files 46 45 -1 Lines 5270 5247 -23 ============================================= + Hits 5245 5247 +2 + Misses 25 0 -25 ``` | [Impacted Files](https://codecov.io/gh/mrc-ide/odin/pull/275?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mrc-ide) | Coverage Δ | | |---|---|---| | [R/generate\_js.R](https://codecov.io/gh/mrc-ide/odin/pull/275/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mrc-ide#diff-Ui9nZW5lcmF0ZV9qcy5S) | `100.00% <100.00%> (ø)` | | | [R/js.R](https://codecov.io/gh/mrc-ide/odin/pull/275/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mrc-ide#diff-Ui9qcy5S) | `100.00% <100.00%> (+0.47%)` | :arrow_up: | | [R/js\_bundle.R](https://codecov.io/gh/mrc-ide/odin/pull/275/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mrc-ide#diff-Ui9qc19idW5kbGUuUg==) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mrc-ide). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mrc-ide)

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