MRC-538: remove use of context/queuer

[richfitz]

This PR removes use of the context and queuer packages from rrq. Doing this will allow us to do two things:

• submit the package to CRAN without worrying about a long dependency chain
• re-implement the bulk submission (lapply - like interface) in a way that fits better with the new environment creation

The bulk of the new code here was directly copied in from context / queuer with simplifications to remove unused code branches. The code in expression.R will change when we do bulk submission, and includes a couple of options that are not (yet) exposed to enqueue. The progress bar bits are all a bit tedious, but nice for interactive use.

[codecov[bot]]

Codecov Report

Merging #9 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff           @@
##           master     #9    +/-   ##
======================================
  Coverage     100%   100%            
======================================
  Files          14     16     +2     
  Lines         849    955   +106     
======================================
+ Hits          849    955   +106

Impacted Files	Coverage Δ
R/rrq_controller.R	100% <100%> (ø)	:arrow_up:
R/expression.R	100% <100%> (ø)
R/worker_run.R	100% <100%> (ø)	:arrow_up:
R/utils.R	100% <100%> (ø)	:arrow_up:
R/worker_messages.R	100% <100%> (ø)	:arrow_up:
R/worker_spawn.R	100% <100%> (ø)	:arrow_up:
R/time.R	100% <100%> (ø)
R/worker.R	100% <100%> (ø)	:arrow_up:

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