crypto: optimize sign.update() and verify.update()

The first commit is #31766.

Second commit:

Use StringBytes::InlineDecoder to decode strings inputs in C++ land instead of decoding them to buffers in JS land before passing them on to the C++ layer. This is what the other update() methods already did.

Third commit:

Factor out the common logic into a template function. Removes approximately six instances of copy/pasted code.