[ latex-backend ] Adding AgdaFixityOp

- The identifiers whose fixity is being declared are now encapsulated
  by \AgdaFixityOp{}s.
- Added corresponding test cases
Merge branch 'master' of https://github.com/agda/agda
Merge branch 'master' of https://github.com/agda/agda
Merge pull request #21 from gallais/master

[ latex-backend ] Adding AgdaFixityOp