Allow workers to parallelise non-package models

[richfitz]

Fixes #211, requires mrc-ide/dust#368 - before merge update branch pins

See the test case and #211 for a motivating example - if a model is created with dust::dust() then workers will fail to work because the alloc method (indeed, all the dust methods) are not found. This is because the model is created in a little fake package known only to the parent session. With the dust PR (mrc-ide/dust#368) we can easily load this fake package after deserialisation in a child process, with this function being a no-op if it is not needed. This PR just calls dust::dust_repair_environment unconditionally after loading the serialised data.

[codecov]

Codecov Report

Merging #215 (53ea96c) into master (1f2410d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #215   +/-   ##
=======================================
  Coverage   99.92%   99.92%           
=======================================
  Files          29       29           
  Lines        2797     2798    +1     
=======================================
+ Hits         2795     2796    +1     
  Misses          2        2           

Impacted Files	Coverage Δ
R/pmcmc_chains.R	100.00% <100.00%> (ø)

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