Modules vignette review (#982)

* Harmonize inline examples, defer all package-related stuff.

* As a preliminary step before reviewing the specific section about packges.
* General refinement to make it easier for a user to follow and run the examples.
* Now all inline examples assume fxx (clearly stated) and explicitly call Module(), to make the examples closer to self-contained.
* Also fixed minor typos.

* Complete World S4 dispatch example.

* World was not already included in the examples above.

* Refine and fix inline module examples, also made runnable.

* Inline examples with R code now have a non-included chunk creating the assumed relevant fx object based on the C++ code in previous named chunk(s).
* The document has gained a parameter `eval` in the YAML header, that can be set to TRUE to optionally run the examples.
* This revealed compilation errors with the std::vector example, fixed with a workaround, to be discussed upon PR.

* Major package section review; cover sourceCpp usage.

* Focus on recommended usage via loadModule with example * General review, also covering exports.
* New section about using the convenient sourceCpp.

* Batch cosmetic refinements and alignment.

* \proglang all the way.
* \textsl{Rcpp modules} everywhere but in their own section.
* Consistent usage of inline C++ elements.

* Convenience wrapper to support evaluation + `results` chunk option.

* Make evaluation even less intrusive.

* Include @eddelbuettel and @coatless PR #982 feedback.

* Removed parameters for evaluating chunks, restored evaluation as before, keeping named Rcpp chunks.
* Finalized comment about resize and push_back workarounds for C++11.
* Referred to Rcpp attributes vignette for sourceCpp.
* Removed mustStart=TRUE (undocumented), the example works without.
* Removed left-over style option chunks not needed.

Author: riccardoporreca