bigi: remove extraneous whitespace
convert: extend test data and include DER integers

These tests failing show that currently toBuffer/fromHex does not handle
negative integers correctly, as toByteArrayUnsigned takes the absolute()
value of the number.
convert: fix *ByteArrayUnsigned impl.
convert: add tests for *ByteArrayUnsigned
convert: rename toByteArraySigned -> toDERInteger

The documentation was exhibiting custom format, not representative of a
DER integer, that was amended.

Tests added using the DER test data.
convert: rework toBuffer/toHex for performance