Speed up and refactor Hints mode

Speed up and refactor Hints mode

This makes most hints show up on screen up to twice as fast as before.
The rest of the hints show up at the same speed as before. Hints
generation has been optimized for the common case, by looking for
markable elements in two passes (if needed and possible). First, we look
for common and fast-to-find elements (such as links); then we look for
everything else. Addresses #409.

Code has been pulled out of marker.coffee, modes.coffee and
hints.coffee, and put into the new marker-container.coffee. hints.coffee
was then renamed to markable-elements.coffee (because all it does now is
finding markable elements).

The concept of tagging elements as "semantic" has been removed. The idea
was to give "unsemantic" elements worse hints (so they wouldn't trump
links, for example). Now, such elements are usually found in the second
pass instead, which gives about the same effect.

`<c-enter>` in Hints mode no longer replaces all hints with new ones for
_all_ elements on screen. Instead, it replaces the hints with hints for
all _unmarked_ elements on screen. Pressing `<c-enter>` again toggles
back to the original hints.

There is a breaking API change for `vimfx.setHintMatcher(hintMatcher)`.
`hintMatcher` no longer receives and returns an object (of the shape
`{type, semantic}`), but instead simply receives and returns the `type`
of the element.