Removed passing of references to objects which are already references

[mirao]

Found by clippy.

E.g. for

self.highlight_primary_keywords(&mut index, &opts, &chars)

you get this warning

warning: this expression borrows a reference (`&filetype::HighlightingOptions`) that is immediately dereferenced by the compiler
   --> src/row.rs:447:64
    |
447 |                 || self.highlight_primary_keywords(&mut index, &opts, &chars)
    |                                                                ^^^^^ help: change this to: `opts`
    |

which is true, because opts is already a reference to &HighlightingOptions