Rewrite of helper functions for dynamic Rd files

Removed geom_aesthetics and stat_aesthetics since they were
not used anywhere else in the codebase

Refactored aesthetics and rd_aesthetics so that aesthetics only
collects the information about a given geom or stat's aesthetics
and rd_aesthetics is responsible for all the formatting of this
into Rd format. Parameters of the two functions are the same, but
aesthetics now returns a data.frame rather than a character vector.

rd_aesthetics now also outputs the default values for aesthetics.

roxygen-like documentation added for aesthetics function (same format
as roxygen documentation, but with just comment prefix rather than
roxygen prefix) as a type of internal documentation.
Fix bug in aesthetics shown by GeomCustomAnn

When a default was -Inf, it would come across as a language
object, not a numeric object. This caused problems as a
component of a list as a column of a data.frame. Running eval
on each element removes the call and makes -Inf numeric.
Fix bug in aesthetics shown by GeomBoxplot and StatBoxplot

When a default was a derived variable (..count..), eval
could not be called on it, so only call eval on those that
are actually calls.
Resurrect TopLevel$find_all()

This function was not used in any (non-commented out) code.
Now works again. only.documented argument dropped because
the objects do not contain their own documentation anymore.
Fix Rd output for aesthetic with no default

Propogated change in aesthetics() where the default is
a character to rd_aesthetics so that the checks are against
a character vector.
Infrastructure to link to geoms that use an aesthetic

geom_with_aesthetic and rd_geom_with_aesthetic in
utilities-help.r and a new Geoms section in the aesthetics
roxygen file which includes build-time lookup of which
geoms recognize a given aesthetic, creating a list and link
to them. re-roxygenized.