Add a BN_num_bits() with zero padded input.

Currently BN_hex2bn() removes the leading zeros, however this will not be
the case in the future.

Fix copy-paste error

Add regress coverage for BN_num_bits()

Add regress coverage for the new behavior of BN_copy() with respect to
flags.

Add regress coverage for BN_num_bits_word()

Add a few missing headers and drop a few unused ones

In bn_test.c include bn_local.h instead of using copy-pasted prototypes.

Flatten structure of libcrypto/bn tests

The bn tests were distributed into three subdirectories rather randomly.
It's cleaner and easier to maintain if all this is in a single directory.
Use consist

Flatten structure of libcrypto/bn tests

The bn tests were distributed into three subdirectories rather randomly.
It's cleaner and easier to maintain if all this is in a single directory.
Use consistent names for the .c files, unify handling of the tests with
the exception of bn_test, which is special.

Discussed with jsing

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