modularize associated iterators

[alejandro-vaz]

the associated iterators of SmallVec are:

• Drain => src/iterators/drain.rs
• IntoIter => src/iterators/mod.rs
• Splice => src/iterators/splice.rs
• ExtractIf => src/iterators/extractif.rs

each of those iterators should be moved into their file with all its methods and implementations

[alejandro-vaz]

this is still free