Closes #576

Kind of in two minds about removing this

In favour of removing it:

• the macro is not necessary any more (im_str!("blah") can just replaced with "blah", and doing this replacement even in a large code base wasn't particularly ardious)
• it's been deprecated since v0.8 which has been out about a year, so chances are most projects that might update to v0.9 would have already removed it to silence the annoying deprecation-warning

In favour of keeping it:

• The macro is relatively self-contained and not much burdent to keep around
• Nostalgia? ( 😛 )