Reduce HTML coverage size by compressing class names

[GalloDaSballo]

Describe the desired feature

From looking at:

echidna/lib/Echidna/Output/Source.hs

Line 87 in 1ef14f0

".executed { background-color: #afa; }" <>

And html reports it seems that echidna uses only 4 classes:
.executed
.neutral
.reverted
.unexecuted

These classes are repeated on each line, contributing to file-size for close to zero advantage

You could rename each class to use it's first letter, this would save file size (and may make the process of generating the report faster)