Add initial sequences package + Make package names consistent

[dnezam]

Implements all definitions from Dafny that do not rely on any (multi)set-related definitions. These will be implemented using separate branches.

Additionally renames the packages to their singular form, and updates folder and file names for consistency.

[dnezam]

The following functions still have a quantifier in their postcondition:

• Remove (2 quantifiers ensuring the rest of the sequence is the same)
• RemoveValue (2 quantifiers ensuring the rest of the sequence is the same)
• Insert (2 quantifiers ensuring the rest of the sequence is the same)
• Reverse (1 quantifier ensuring the "reversed" property)
• Repeat (1 quantifier ensuring the contents of the sequence)
• Max (1 quantifier ensuring that the maximum is indeed the maximum)
• Min (1 quantifier ensuring that the minimum is indeed the minimum)

Do we want to factor out these quantifiers like we did for IndexOf and IndexOfLast? Additionally, it may be useful to put the 2 quantifiers ensuring that a sequence is the same besides at one index into a separate function, since it is reused multiple times.