Performance: 'Minimal' layout mode

[bryphe]

This is a change to add an alternate layout mode - a streamlined layout when portions of the tree just need to be positioned absolutely relative to the direct parent or via transforms. This saves expensive layout calculations.

This adds a style layoutMode with the settings of Default or Minimal. This is an advanced-use-case setting (ie, trying to squeeze extra perf out of the layout system in constrained cases), but shouldn't be something you need day-to-day. In Onivim2, I'm experimenting with rendering the lines in the minimap with this strategy (although longer term - would like to render directly to a pixel buffer!)

This also adds an initial set of benchmarks via 'reperf'`, and highlights the difference between the standard layout and the 'minimal' layout strategy:

+----------------------------------------------------------------------------------------------------------------------------------------------------------+
|  BENCHMARK                                                   |  TIME             |  MINOR GC  |  MAJOR GC  |  MINOR ALLOC  |  PROMOTED   |  MAJOR ALLOC  |
|--------------------------------------------------------------+-------------------+------------+------------+---------------+-------------+---------------|
|  Node: create single node                                    |  0.0019953250885  |  5         |  0         |  1090240      |  374        |  374          |
|--------------------------------------------------------------+-------------------+------------+------------+---------------+-------------+---------------|
|  Layout: layout single node                                  |  0.079295873642   |  5         |  0         |  1260481      |  421        |  421          |
|--------------------------------------------------------------+-------------------+------------+------------+---------------+-------------+---------------|
|  Layout: layout node tree (4 deep, 4 wide)                   |  3.63596487045    |  2327      |  865       |  458726216    |  125113447  |  125113447    |
|--------------------------------------------------------------+-------------------+------------+------------+---------------+-------------+---------------|
|  Layout: layout node tree (4 deep, 4 wide) - minimal layout  |  0.187062501907   |  53        |  0         |  13742150     |  2482       |  2482         |
+----------------------------------------------------------------------------------------------------------------------------------------------------------+

This should help lower the layout cost when re-rendering Onivim 2, although more work is still needed (in my current benchmarks - lowered the cost ~50%). We should still explore generally improving the layout nodes - regenerating the layout tree is very costly!