Corrected stdlib_bitsets.md

Implemented most of the suggestions of Jeremie Vandenplas:
1. Rephrased the first paragraph adding a definition of bits and changed
'a sequence' to 'as a seqauence' and 'a subset' to 'as a subset'
2. Changed 'constants all integers' to 'integer constants'
3. Changed 'up 64 bits' to 'up to 64 bits'
4. Changed i'th to i-th
5. Made the summary of operations into a bulleted list
6. Moved the discussion of assignments to later in the text
7. Merged discussions of result character and result value for functions/operators
8. Remove `if ( bits(set0) /= 33)...`
9. Added commas
10. Changed `Result = ...` to `result = ...`
11. Changed state type of the result for the value function from logical to integer

[ticket: X]

Author: wclodius2