Implements PKCS #7 padding. This implements the PKCS #7 padding scheme. This scheme is defined in RFC 5652, section 6.3. Strings will be padded out to multiples of the block size; for example, a 5-byte string with an 8-byte block size will have three bytes of padding added. If the length is already a multiple of the block size, an entire block size worth of padding is added. The padding bytes are all set to the number of padding bytes. Returning to the previous example, the padding string would be three bytes of the byte 0x03. Unpadding checks the length and padding, and if this is valid, strips off the padding.