Improve traits and document them

[faassen]

I do the following:

• document the search method on the FMIndexBackend trait

• make the len() method public on the FMIndexBackend trait as I think it may be useful to end-users.

• Introduce a HeapSize trait so we can get the heap size of an index generically

• Introduce API tests that exercise the API through the traits, to make sure I got them right (it's easy to get into infinite recursion; only fixed once I refined the trait bounds for Character and Converter).

• Remove the SearchIndex struct in favor using the FMIndexBackend trait as discussed.

[ajalab]

Thanks!