Fix IsabelleTypeArgs parser

[Chris Warburton]

Hit this error:

Getting args of 'Global6772616d6d6172732f7061636b7261745f756e616d6269675061636b7261742e736d7432546f6b Global6772616d6d6172732f7061636b7261745f756e616d6269675061636b7261742e736d74326c697374 => (Global6772616d6d6172732f7061636b7261745f756e616d6269675061636b7261742e736d7432546f6b Global6772616d6d6172732f7061636b7261745f756e616d6269675061636b7261742e736d74326c697374) Global6772616d6d6172732f7061636b7261745f756e616d6269675061636b7261742e736d74326c697374 => Global437573746f6d426f6f6c Global6772616d6d6172732f7061636b7261745f756e616d6269675061636b7261742e736d74326c697374'
dz2db435pdf5vb2r9yfd5mb2wfbj5pl6-IsabelleTypeArgs.hs: Couldn't parse 'Global6772616d6d6172732f7061636b7261745f756e616d6269675061636b7261742e736d7432546f6b Global6772616d6d6172732f7061636b7261745f756e616d6269675061636b7261742e736d74326c697374 => (Global6772616d6d6172732f7061636b7261745f756e616d6269675061636b7261742e736d7432546f6b Global6772616d6d6172732f7061636b7261745f756e616d6269675061636b7261742e736d74326c697374) Global6772616d6d6172732f7061636b7261745f756e616d6269675061636b7261742e736d74326c697374 => Global437573746f6d426f6f6c Global6772616d6d6172732f7061636b7261745f756e616d6269675061636b7261742e736d74326c697374': "(unknown)" (line 1, column 174):
unexpected "="
expecting "(" or ")"
builder for â??/nix/store/8iawnykaa1pqspdnzr03nq2ivsf3m70v-datatypes-filter-test.drvâ?? failed with exit code 1

Haven't narrowed it down yet, but I've added some more tests to
IsabelleTypeArgs.hs, including QuickCheck tests.