Add simple parser benchmark

[Kijewski]

No description provided.

[Kijewski]

As a test, I sped-up the parsing of nodes a bit

librustdoc/all          time:   [355.54 µs 356.38 µs 357.32 µs]
                        thrpt:  [39.520 MiB/s 39.623 MiB/s 39.718 MiB/s]
                 change:
                        time:   [-18.234% -16.825% -15.541%] (p = 0.00 < 0.05)
                        thrpt:  [+18.401% +20.229% +22.301%]
                        Performance has improved.

librustdoc/item_info    time:   [5.0121 µs 5.0287 µs 5.0523 µs]
                        thrpt:  [31.145 MiB/s 31.292 MiB/s 31.395 MiB/s]
                 change:
                        time:   [-23.798% -22.817% -21.894%] (p = 0.00 < 0.05)
                        thrpt:  [+28.031% +29.562% +31.230%]
                        Performance has improved.

librustdoc/item_union   time:   [29.414 µs 29.473 µs 29.544 µs]
                        thrpt:  [33.410 MiB/s 33.490 MiB/s 33.557 MiB/s]
                 change:
                        time:   [-16.596% -14.874% -13.411%] (p = 0.00 < 0.05)
                        thrpt:  [+15.489% +17.473% +19.898%]
                        Performance has improved.

librustdoc/page         time:   [155.88 µs 156.54 µs 157.19 µs]
                        thrpt:  [39.392 MiB/s 39.557 MiB/s 39.723 MiB/s]
                 change:
                        time:   [-12.364% -11.542% -10.749%] (p = 0.00 < 0.05)
                        thrpt:  [+12.044% +13.048% +14.108%]
                        Performance has improved.

librustdoc/print_item   time:   [20.818 µs 20.840 µs 20.866 µs]
                        thrpt:  [45.247 MiB/s 45.304 MiB/s 45.351 MiB/s]
                 change:
                        time:   [-12.666% -11.880% -10.936%] (p = 0.00 < 0.05)
                        thrpt:  [+12.278% +13.482% +14.503%]
                        Performance has improved.

librustdoc/short_item_info
                        time:   [18.014 µs 18.028 µs 18.047 µs]
                        thrpt:  [50.202 MiB/s 50.253 MiB/s 50.293 MiB/s]
                 change:
                        time:   [-16.748% -15.854% -14.914%] (p = 0.00 < 0.05)
                        thrpt:  [+17.529% +18.842% +20.118%]
                        Performance has improved.

librustdoc/sidebar      time:   [34.118 µs 34.204 µs 34.316 µs]
                        thrpt:  [35.961 MiB/s 36.079 MiB/s 36.170 MiB/s]
                 change:
                        time:   [-21.998% -21.094% -20.103%] (p = 0.00 < 0.05)
                        thrpt:  [+25.161% +26.732% +28.202%]
                        Performance has improved.

librustdoc/source       time:   [18.575 µs 18.599 µs 18.628 µs]
                        thrpt:  [39.574 MiB/s 39.637 MiB/s 39.688 MiB/s]
                 change:
                        time:   [-13.313% -12.557% -11.734%] (p = 0.00 < 0.05)
                        thrpt:  [+13.294% +14.360% +15.358%]
                        Performance has improved.

librustdoc/type_layout_size
                        time:   [7.6326 µs 7.6403 µs 7.6503 µs]
                        thrpt:  [35.403 MiB/s 35.449 MiB/s 35.485 MiB/s]
                 change:
                        time:   [-28.176% -27.557% -26.758%] (p = 0.00 < 0.05)
                        thrpt:  [+36.533% +38.039% +39.229%]
                        Performance has improved.

librustdoc/type_layout  time:   [50.909 µs 50.962 µs 51.035 µs]
                        thrpt:  [52.752 MiB/s 52.828 MiB/s 52.883 MiB/s]
                 change:
                        time:   [-10.061% -8.7841% -7.6337%] (p = 0.00 < 0.05)
                        thrpt:  [+8.2646% +9.6300% +11.186%]
                        Performance has improved.

[djc]

This is great, thanks!