Reimplement decorations as an interval tree

[alexdima]

This PR reimplements decorations in the editor buffer as an interval tree which stores relative offsets. The goal is to make inserting, removing and editing be a lot faster when dealing with a large number of decorations. Memory usage savings are also a nice win.

This should help with increasing the limits for #496 and #1110

Further optimizations (on top of an interval tree):

• storing two interval trees, one with decorations that show up in the overview ruler, one with decorations that do not. The reason is that the overview ruler paints all the decorations, not just the ones in the viewport.
• the decorations overview ruler has been reimplemented to avoid allocations almost entirely.
• fetching decorations to render in the viewport now eliminates hidden ranges (i.e. folded ranges) and avoids visiting decorations that would not be visible anyways.
• the find widget, when having more than 1k results, will approximate overview ruler decorations and will create a separate set of decorations for the overview ruler.

checker.txt + Bracket Pair Colorizer ~ 38,177 decorations

	Before	After
removing+adding	194.5ms (52.5+56.3+85.7)	55.7ms (25.4+14.8+15.5)
typing \n (_pushEditOperations)	148.7ms	1.6ms
model mem usage	3,865,672	3,858,468
model mem usage with decorations	40,826,928	6,650,196

jquery.js + Search for e + no find limit ~ 24,034 decorations (22,974 find matches)

	Before	After
adding	230.3ms	38.9ms
removing	759.6ms	23.9ms
typing \n (_pushEditOperations)	33.1ms	0.7ms
model mem usage	2,667,304	2,630,636
model mem usage with decorations	15,269,076	4,228,524

[eamodio]

@alexandrudima THIS IS AWESOME! I'm seeing a great improvement in decoration application & removal -- and even better it also seems stable (performance-wise) -- before adding a LOT of decorations would be slow-ish but ok, but removing them would sometimes get worse and worse over time.

Thank you!!!

[lkytal]

Brillant work.