Vec <-> BTreeMap conversion not available in no_std mode

[jonasbb]

Someone on Discord pointed out that Seq <-> Map conversions don't work under no_std, given that the marker types Vec, BTreeMap, HashMap are only available with alloc/std.

#[cfg(feature = "alloc")]
tuple_seq_as_map_arr!([(K, V); N], BTreeMap<KAs, VAs>);
#[cfg(feature = "std")]
tuple_seq_as_map_arr!([(K, V); N], HashMap<KAs, VAs>);

• Introduce Map and List/Seq types as replacements available in no_std.
• Implement the required conversions.
  • The conversions might be able to use IntoIterator/FromIterator instead of hard-coding certain combinations. This could bring it closer to the older tuple_list_as_map behavior.
• Update existing examples to slowly migrate to these new types.

Open questions:

• Is List or Seq the better name? Serde uses seq.
• Map might conflict with type aliases or import renames. But are there alternatives?