Add multi-part operations for asymmetric signature

[athoelke]

This adds the new operations for multipart sign and multi-part verify for asymmetric signatures (previous discussions suggested these should be ditinct types).

• Updated the Functionality chapter to indicate the availability of multi-part operations for signature (and added two missed additions in 1.4).
• Extended the introduction to Asymmetric Signatures to describe the multi-part operations, and how they extend multi-part capability to message-signature algorithms.
• Added/modified text in signature algorithms to reflect their compatibility with the mluti-part operations.
• Modified the Notes in the single-part functions which recommended the use of a multi-part hash operation for fragmented messages.
• Add new operation types and functions.

Fixes #319

[athoelke]

This needs to wait for the 1.4.1 publication to be finalised.

One aspect of the proposed API that might be debatable:

• I've added context parameters to the setup functions, which can be zero-length (as per the _with_context single part functions). This seems to be a better approach than adding an additional, optional to call, xxx_set_context()function to each operation - which adds additional states and complexity to the implementation.