make tinyplot_add compatible with do.call(tinyplot)

[zeileis]

Fixes #499

The .last_call of the tinyplot() generic is now determined by comparing the functions called in sys.calls() rather than their names as suggested by @FlorianSchwendinger.

This makes it compatible with usage in do.call() or when using a custom copy of the generic.

do.call(tinyplot, list(x = 1:5, y = sin(1:5)))
do.call(tinyplot_add, list(type = "h"))

myplot = tinyplot
myplot_add = tinyplot_add
myplot(x = 1:5, y = sin(1:5))
myplot_add(type = "h")

[grantmcdermott]

Thanks @zeileis, this looks good to me. One minor nit and then we can can merge once #473 is shepherded through. (I want to wait for that, since it's a big change that will cause some adjustments across most of the codebase.)