More consistent legacy color conversion

When a given "legacy color" argument is not in fact a recognised color
name, `legacy_color` should return nil, and indeed it does when the
argument is a string.

However, the color symbol method will just return the symbol argument if
it is not recognised, creating an inconsistency in the behaviour.
Instead, it should return nothing, as the string case does.

This change is made, and a little of the surrounding code rearranged for
clarity.

src/legacy.jl

@@ -8,6 +8,15 @@ module Legacy
 
 using ..StyledStrings: SimpleColor, Face, loadface!, face!
 
+"""
+    legacy_color(color::Union{String, Symbol, Int})
+
+Attempt to obtain a `SimpleColor` for a "legacy" color value `color`.
+
+When this is not possible, `nothing` is returned.
+"""
+function legacy_color end
+
 """
 A mapping from 256-color codes indicies to 8-bit colours.
 """
@@ -51,6 +60,19 @@ const ANSI_256_COLORS =
          0x8a8a8a, 0x949494, 0x9e9e9e, 0xa8a8a8, 0xb2b2b2, 0xbcbcbc, 0xc6c6c6,
          0xd0d0d0, 0xdadada, 0xe4e4e4, 0xeeeeee])
 
+legacy_color(color256::Int) = get(ANSI_256_COLORS, color256+1, nothing)
+
+"""
+A list of all named colors recognised, including both the old `light_*` and new
+`bright_*` named colors.
+"""
+const NAMED_COLORS =
+    (:black, :red, :green, :yellow, :blue, :magenta, :cyan, :white,
+     :bright_black, :grey, :gray, :bright_red, :bright_green, :bright_yellow,
+     :bright_blue, :bright_magenta, :bright_cyan, :bright_white, :light_black,
+     :light_red, :light_green, :light_yellow, :light_blue, :light_magenta,
+     :light_cyan, :light_white)
+
 """
 A mapping from old named colours to the new names, specifically from `light_*`
 to `bright_*`.
@@ -65,24 +87,13 @@ const RENAMED_COLORS = Dict{Symbol, Symbol}(
     :light_cyan    => :bright_cyan,
     :light_white   => :bright_white)
 
-legacy_color(color::Symbol) = SimpleColor(get(RENAMED_COLORS, color, color))
-legacy_color(color256::Int) = get(ANSI_256_COLORS, color256+1, nothing)
-
-"""
-    legacy_color(color::Union{String, Symbol, Int})
-
-Attempt to obtain a `SimpleColor` for a "legacy" color value `color`.
+legacy_color(color::Symbol) =
+    if color in NAMED_COLORS
+        SimpleColor(get(RENAMED_COLORS, color, color))
+    end
 
-When this is not possible, `nothing` is returned.
-"""
 function legacy_color(color::String)
-    namedcolours = ("black", "red", "green", "yellow", "blue", "magenta",
-                    "cyan", "white", "bright_black", "grey", "gray",
-                    "bright_red", "bright_green", "bright_yellow",
-                    "bright_blue", "bright_magenta", "bright_cyan",
-                    "bright_white", "light_black", "light_red", "light_green",
-                    "light_yellow", "light_blue", "light_magenta", "light_cyan",
-                    "light_white")
+    namedcolours = String.(NAMED_COLORS)
     if color in namedcolours
         legacy_color(Symbol(color))
     elseif 0 <= (color256 = something(tryparse(Int, color), -1)) <= 255