Allow making instances of BitVec with B≠u32

[Columbus240]

Previously it was not possible to construct such instances of BitVec other than via the Default trait. Using the *_general methods, one can construct instances of BitVec with arbitrary B. See the examples in the doc comments for the syntax.
In particular the with_capacity function is now available for arbitrary B.

I'm not certain about the following parts and would like some input or changes by the maintainer:

• The naming of the more general functions. I'm not sure whether the names I gave are appropriate.
• The documentation. Currently I just copy-paste and adapt the already existing comments, but do not mention the difference between corresponding functions.
• The existing code for u32 made use of the |= operator (ops::BitOrAssign). I didn't want to change this code, so I added BitOrAssign as requirement for the BitBlock trait. By replacing a |= b with its equivalent form a = a | b, this requirement could be removed.

I haven't checked, whether there is a similar situation in other libraries.

[pczarn]

Not quite sure how to change the API.

[pczarn]

I have some ideas, but no spare time, please ask me before any work on this.