This turns error messages like
gf: too few bytes. Failed reading at byte position 1
gf: /some/path/somefile.gfo: too few bytes. Failed reading at byte position 1
but a better fix would be to ignore bad .gfo files and compile from source.
The problem is the way this decision is made in
GF.Compile.ReadFiles.selectFormat...
This speeds up the compilation of PhrasebookFin.pgf by 12%, mosly by speeding
up calls to lookupModule in calls from lookupParamValues, in calls
from allParamValues.
The invariant "modules are stored in dependency order" is no longer respected!
But the type MGrammar is now abstract, making it easier to maintain this or
other invariants in the future.
