symbol legend tests; fix symbol scale hint; symbol identity scale

[mbostock]

This…

• Adds support for the identity scale type for the symbol scale, allowing channels to return symbol type names or objects.
• Like automatic color identity scale #673, automatically chooses the identity scale type for the symbol scale if all associated values are valid symbol types. Values are pulled from the domain if present, and otherwise from channels.
• Fixes a bug whereby if a dot’s fill and symbol channels have different definitions, the symbol hint’s fill was undefined even though the dot is filled; now the hint fill is set to currentColor such that the symbol scale will chose the appropriate default range.
• Fixes Plot.legend to allow a combined color + symbol legend by specifying both scales.
• Fixes Plot.legend to allow the fill and stroke option to generate a corresponding hint for the symbol scale. This ensures that the scale will have the expected default range. An appropriate hint is also generated for the combined color + symbol legend.
• Renames the “x” symbol to “times” to avoid confusion. For example, fill: "x" means to fill using the field named “x”, but previously symbol: "x" means to use the (constant) symbol in the shape of an X.
• Adds snapshot test for symbol legends.

We should probably rename d3.symbolX to d3.symbolTimes in d3-shape, too, (keeping the old name as a deprecated alias to avoid backwards incompatibility). But that’s not urgent.