Add native SIR filter example

[richfitz]

This PR removes dust as a dependency and starts the exploration of a proper rng interface. There's quite a bit in here as a result. But the main thing that we're trying to do is to build a well-behaved stochastic model that can eventually unblock #68. I don't think that this has everything (you need grouped structure too, right @edknock?) but this gets us most of the way.

The monty_rng interface is not suitable for creating models because we can't use the streams in the same way as we do from dust. If we have n values in size and prob and n streams in rng_state I want to be able to do Binomial(size, prob, rng_state) and have the first stream use the first size and prob, the second stream use the second etc. This is not implemented in monty_rng and this means we have no way to draw random numbers in a way that we can build models that look like dust but without using it.

Here I've sketched out a couple of accessor functions (see the 2 files in both R/ and src/); these allow access in this way. Then we build a particle filter by hand (this is tedious and was done following the manual implementation in the dust2 helpers: https://github.com/mrc-ide/dust2/blob/main/tests/testthat/helper-dust.R#L5). With that I have managed to replace all the dust code and replaced it with this model.

The next step will be to make this work with multiple parameter sets, which I'll do in the next PR.

Merge after #84, contains those commits

[codecov]

Codecov Report

Attention: Patch coverage is 97.72727% with 1 line in your changes missing coverage. Please review.

Project coverage is 99.45%. Comparing base (ab2368f) to head (4f84a3c).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/random2.cpp	97.22%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #85      +/-   ##
==========================================
- Coverage   99.46%   99.45%   -0.02%     
==========================================
  Files          64       66       +2     
  Lines        4889     4933      +44     
==========================================
+ Hits         4863     4906      +43     
- Misses         26       27       +1     

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[weshinsley]

Talked through yesterday - all looks good - and we'll mop up the coverage issues a bit later...