Naming inconsistency: `len` and `nbits` are used interchangeably

[Jujumba]

In general, I do think we have to rename nbits to len in the public API wherever possible. some examples I found so far:

• panic message of the insert function
• debug output field is named nbits, which matches the type definition but is confusing, because the function to get length is called len

[pczarn]

First case, yes of course, let's change it, and the second case seems okay as it is.

[pczarn]

The panic message could be improved.

The output of Debug is not defined as stable or strictly useful for the user, I believe.

[Jujumba]

The output of Debug is not defined as stable or strictly useful for the user, I believe.

Though I would personally find it more confusing, but fine

[Jujumba]

First case, yes of course, let's change it, and the second case seems okay as it is.

Opened #117

[pczarn]

Thanks! Closing as fixed

[Jujumba]

We should also rename the nbits argument in with_capacity and from_elem to capacity and len respectively

@pczarn