Add Text instance for CompilerFlavor and CompilerId

This is a tad subtle since we actually have two ways of parsing
compiler flavours. One expects, or at least allows lower case names
like "ghc" and "nhc98", the other uses Read/Show which gives "GHC"
and "NHC". What we're doing here is only changing the first variety.
The cases where we parse using Read (and display using Show) are not
changed. At some point we'll switch those over to the more liberal
parser, but not yet as we don't want to cause compatibility problems.