Adding scalar expressions to views in addition to expression columns.

[robmccoll]

The idea would be to calculate these first and store them in a map on the data table under the view accessible to the column expressions either via a function or by exposing symbols. This would allow for things like pre-calculating a bucket width based on min/max, calculating standard deviation and mean and then creating a column with std deviations from the mean for a given value, delta to max or min value, etc. It could also be used with a simple table plugin to display a panel of user-defined descriptive statistics.

I've been trying to follow the model of column expressions and thread this through everything. It should be exposed up through to the Emscripten / JS layer enough to define the expressions and have them computed, but I haven't wrapped my head around the Rust + Less components enough to display and edit them to really poke at it and test it.