[crypto] Add bigint_ntoa() for transcribing big integers

[crypto] Add bigint_ntoa() for transcribing big integers

In debug messages, big integers are currently printed as hex dumps.
This is quite verbose and cumbersome to check against external
sources.

Add bigint_ntoa() to transcribe big integers into a static buffer
(following the model of inet_ntoa(), eth_ntoa(), uuid_ntoa(), etc).

Abbreviate big integers that will not fit within the static buffer,
showing both the most significant and least significant digits in the
transcription.  This is generally the most useful form when visually
comparing against external sources (such as test vectors, or results
produced by high-level languages).

Signed-off-by: Michael Brown <mcb30@ipxe.org>