This patch release cleans up warnings during compilation.

Fixes:

• Broken composition with an empty list in the path (#19)

Bug-fixes

• Fixes using lenses to index into empty lists.
  • Fixes #16; thanks @adleroliveira!
• Fixes using non-numeric lenses to index into non-keyword lists.

Enhancements (non-breaking)

• Add pipeline-able versions of Focus.view/2, Focus.set/3, Focus.over/3