Restructure data structures section

[kilianmh]

Two questions:

• Should data structures separate custom data structures from inbuilt ones?
• Should we create subsections for each inbuilt data structure?

The idea is that you get a swift overview about tools to deal with certain inbuilt data structures and separate it clearly from custom data structures, such as hash-set.

Here we could have subsections for each data structure and put corresponding libraries, as well as link parts of bigger utility libraries e.g. alexandria sequences.

As far as I know the inbuilt data structures would be (probably some are missing):

• sequences
  • lists
    • alist
    • plist
    • tree
    • set
• array
  • vectors (here or in sequences?)
• hash-table
• structures (should this be in data structures section?)
• classes (should this be in data structures section?

[vindarel]

it's currently a big section with many different libraries, we can probably separate them better in "general purpose", "algorithms", more specialized… if those in-built categories will be helpful remains to be seen :thinking gif: I prefer such a split for learning purposes, in a tutorial like the Cookbook, here we should focus on awesome libraries. Is there a need to enhance the Cookbook too?