[WAIT] When processing a route, use most specific mount paths first

[schloerke]

Fixes #734
Fixes #773

• More specific mount paths will be found by using this order

Reprex:

pr <- pr() %>% 
  pr_mount("/aaa", pr()) %>%
  pr_mount("/aaa/bbb",
           pr() %>% 
             pr_get("/hello", function() "hi")
  ) %>% pr_run()

... Which mount processes /aaa/bbb/hello?

• Before pr: /aaa
• With pr: /aaa/bbb

PR task list:

• Update NEWS
• Add tests
• Update documentation with devtools::document()

Questions:

• Is it ok to return different order for pr$mounts?
  • Now it is alpha sorted. Before, it was the order in which they were added.
  • I think this new order is reasonable as it reflects the processing order.

Answer: No. Keep pr$mounts the same

[schloerke]

Suggested change

	mnts[sort(names(mnts), decreasing = FALSE)]
	mnts[order(names(mnts), decreasing = FALSE)]

[schloerke]

Suggested change

	# * If `/aaa/bbb` mount does not support `/aaa/bbb/foo`, then try mount `/aaa`.
	# * If `/aaa/bbb` mount does not support `/aaa/bbb/foo`, then try mount `/aaa` with route `/bbb/foo`.

[schloerke]

Suggested change

	# order the mounts so that the more specific mount paths are ahead of the less specific mount paths
	# Add the mount to the original mount order
	private$mnt_order <- c(private$mnt_order, path)
	# Order the mounts so that the more specific mount paths are ahead of the less specific mount paths

[schloerke]

Suggested change

	mnts <- private$mnts
	if (length(mnts) == 0) return(mnts)
	mnts[sort(names(mnts), decreasing = FALSE)]
	private$mnts[private$mnt_order]

[schloerke]

This change is deemed too dangerous. Maybe approach this with pr_merge() (##749) instead?

cc @cpsievert