Lib: simplify `str` using lifetimed sub-strings?

[kud1ing]

My understanding is, that we don't need separate searching functions like all_between anymore, which operate on sub-strings via start/end indices.

I think the same functionality could be achieved by composing methods operating on lifetimed sub-strings (slices?).
E.g. something like all(substr(foo, 0, 5), foo()) instead of all_from.
If this is true, we could get rid of quite some functions and test code.

Candidates for removal:

• all_between
• any_between
• count_chars
• find_between
• find_char_between
• find_char_from
• find_from
• find_str_between
• find_str_from
• rfind_between
• rfind_char_between
• rfind_char_from
• rfind_from

Some more userfriendly substr()-like functions should be added, like between() and from(), so that one could write all(between(...), ...) or find(from(...), ...).