Change byte range formatting

[hackerfactor]

As discussed in #4, my initial definition for the byte range parameter (b=) contains some ambiguities:

• Hyphens are used for both the byte range and for math. E.g., b=p-(s-50). The recommendation is to use hyphens for math and tilda for range: b=p~s-50. This makes the parenthesis unnecessary.
• There should be a variable for the beginning and end of the file: b and e. Such as b=b~s,s~e for the entire file (excluding the signature).
• The s literal has two meanings. On the left-side of the range, it refers to the start of the signature. One the right-side, it refers to the end of the signature. The proposal is to use two different literals for the start and end of the signature. This hasn't been resolved yet.

[named-dodo]

The current updated format plan as far as i am aware:
Example string is b="F~S-100,S-50~s,s+10~f"

1. Split on , to get an array of ranges: F~S-100 / S-50~s / s+10~f
2. Split on ~ to get the begin and end part of the range: s+10~f => s+10 / f
3. A range part must start with a letter to set a base value:
   F for begin of file
   f for end of file
   S for begin of signature
   s for end of signature
   P for begin of previous signature
   p for end of previous signature
4. Each range part can optionally have an offset: a + or - followed by a decimal number of bytes.

The default value would be b="F~S,s~f", meaning from begin of file to begin of signature, from end of signature to end of file.
This means that everything except for the signature is signed. (as you can't sign the signature itself.)

[hackerfactor]

I like that a lot. For me, it's as readable and makes sense.

[hackerfactor]

Specs updated to match.