Fix #87: round-trip problem for .chi files

The Show instance for Ident in language-c changed from version 0.4.2
to later versions, breaking the round-tripping of .chi files used for
dealing with {#import ...#} binding hooks (we use a custom Read
instance for Ident to read the .chi files back).  @garetxe provided a
new custom Read instance that works with more recent language-c
versions.