AgoraLayout: Change colors to be more gradual

[Geod24]

Use a 'final switch' to ensure all cases are handled;
Make Fatal and Error red, freeing up magenta;
Make Debug magenta, and switch colors for trace (formerly Cyan)
and verbose (formerly Blue) so that the color scheme follows
the same order as the natural spectrum of visible light.

Piggybacking on this little test code:

Δ source/agora/utils/Log.d
       host: null,
    };

-    const before = GC.stats();
-    layout.format(event, dg);
-    const after = GC.stats();
-    assert(result == "2018-04-30 00:42:42,420 \u001b[33mWarn\u001b[0m [Barney] - Have you met Ted?");
-    assert(before == after);
+    // const before = GC.stats();
+    // layout.format(event, dg);
+    // const after = GC.stats();
+    // assert(result == "2018-04-30 00:42:42,420 \u001b[33mWarn\u001b[0m [Barney] - Have you met Ted?");
+    // assert(before == after);
+
+    import std.stdio;
+
+    for (size_t i; i < LogLevel.None; ++i)
+    {
+        result.length = 0;
+        assumeSafeAppend(result);
+        event.level = cast(LogLevel) i;
+        layout.format(event, dg);
+        writeln(result);
+    }
}

We had this before:

After this PR we'll have this:

If anyone has preference on another color scheme let me know. I was mostly bothered by Debug being hard to read on a regular terminal.

[codecov[bot]]

Codecov Report

Merging #3227 (a262676) into v0.x.x (52198f2) will increase coverage by 58.05%. The diff coverage is 41.66%.

@@             Coverage Diff             @@
##           v0.x.x    #3227       +/-   ##
===========================================
+ Coverage   29.46%   87.51%   +58.05%     
===========================================
  Files         265      166       -99     
  Lines       27046    16963    -10083     
===========================================
+ Hits         7970    14846     +6876     
+ Misses      19076     2117    -16959     

Flag	Coverage Δ
integration	34.24% <41.66%> (+4.77%)	:arrow_up:
unittests	87.42% <41.66%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
source/agora/utils/Log.d	62.26% <41.66%> (+18.62%)	:arrow_up:
source/agora/common/VibeTask.d	0.00% <0.00%> (-86.67%)	:arrow_down:
source/agora/common/FileBasedLock.d	0.00% <0.00%> (-72.73%)	:arrow_down:
source/agora/network/RPC.d	1.51% <0.00%> (-59.85%)	:arrow_down:
source/agora/network/VibeManager.d	1.42% <0.00%> (-44.29%)	:arrow_down:
source/agora/node/Runner.d	0.00% <0.00%> (-42.23%)	:arrow_down:
source/scpd/scp/SCPDriver.d	60.00% <0.00%> (-15.00%)	:arrow_down:
source/agora/consensus/Quorum.d	97.09% <0.00%> (-0.28%)	:arrow_down:
source/scpd/scp/SCP.d	100.00% <0.00%> (ø)
source/agora/crypto/Crc16.d	100.00% <0.00%> (ø)
... and 311 more

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