Encode signed integers correctly

[aryan-25]

Motivation:

Currently, positive signed integers whose minimal big-endian representation exactly fills N bytes (neededBits % 8 == 0) have their top bit misread as the DER sign bit. Int64(128) is therefore encoded as [0x80] and decoded back as -128. We should fix this.

Modifications:

• Updated the FixedWidthInteger.neededBytes implementation to return bytes + 1 for positive signed integers in this case, causing IntegerBytesCollection to emit the required 0x00 sign byte.
• Added associated test cases.

Result:

Positive signed integers whose minimal big-endian representation exactly fills N bytes (neededBits % 8 == 0) are now encoded correctly.