provide class for suspension of RNG synchronization

[kevinushey]

This PR provides a class, SuspendRNGSynchronizationScope, which can be used to signal that throughout the lifetime of that object Rcpp should not attempt to synchronize the R and C RNG states.

This tool effectively allows users to opt-in to the 'old' Rcpp RNG behavior by creating an instance of this class where desired. This could be used to fix, for example, eddelbuettel/rcppde#14 (comment).

The intention would be for RcppDE to construct an instance of SuspendRNGSynchronizationScope pretty early in the DEOptim_impl implementation.

FWIW, I tested this fix with a locally built copy of RcppDE and it does fix the issue -- I can send a PR if we find that this change looks good.

[codecov-io]

Codecov Report

Merging #862 into master will decrease coverage by 0.15%.
The diff coverage is 50%.

@@            Coverage Diff            @@
##           master    #862      +/-   ##
=========================================
- Coverage   90.46%   90.3%   -0.16%     
=========================================
  Files          70      70              
  Lines        3251    3261      +10     
=========================================
+ Hits         2941    2945       +4     
- Misses        310     316       +6

Impacted Files	Coverage Δ
src/Rcpp_init.cpp	100% <100%> (ø)	⬆️
src/api.cpp	92.77% <40%> (-7.23%)	⬇️

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