Restructure suffix array module

[ajalab]

In #48, I proposed moving sais module to suffix_array.

It's not obvious for readers that the module and function name sais computes a suffix array. Maybe it should be something like suffix_array instead.
#48

However, suffix_array module already exists and consists of the following elements.

• trait for search index (HasPosition)
• methods and structs for sampled suffix arrays (sample, SuffixOrderSampledArray)

It's not so nice to colocate SA-IS related methods in suffix_array with these elements. Therefore, this PR moves the existing suffix_array module elements to different locations beforehand.

• the trait for search index (HasPosition) will be moved to iter module, where other traits for search index (FMIndexBackend, HeapSize) exist. We may also want to consider restructuring the iter module, but this can be done in another work.
• methods and structs for sampled suffix arrays (sample, SuffixOrderSampledArray) will be moved to a new module suffix_array::sample. This is not exposed to crate users.

After that, we can move sais module to suffix_array::sais.

[faassen]

So if I understand it correctly this doesn't yet contain any changes to sais itself. I spent a moment looking for suffix_array::sais but I take it you plan to move that later. Looks good!

[ajalab]

Right! Let me create another PR to move sais module to suffix_array.