Add inc_len_varint function to ngtcp2_crypto.c

Move template.h include to client.cc

Move ngtcp2_range.h into ngtcp2_gaptr.c

Remove unused assert.h includes

Check mmap's return value

Remove unused ngtcp2_macro.h includes

Remove redundant Buffer::bufsize()

Previously, when mmap() failed the client example crashed deep inside ngtcp2 on trying to access address 0xffffffffffffffff. This can happen for example if `--data` specifies a directory.

This commit moves the include of template.h into client.cc as it does
not seem to be used any longer by client.h.

Commit a7d6c8c4 ("Add
ngtcp2_acked_stream_data_offset callback") removed the field streambuf_
which was using the user-defined literal `_k` from template.h:

std::array<uint8_t, 32_k> streambuf_;

The header ngtcp2_gaptr.h does not use anything from ngtcp2_range.h and
this commit suggests moving the include to ngtcp2_gaptr.c instead.

This commit contains a suggestion to add a static function to reduce
some code duplication in ngtcp2_encode_transport_params.

It's now identical to size().

Remove redundant Buffer::head

`head` always equals `begin`. A subsequent diff can further simplify Buffer by removing bufsize(), which is now identical to size().

Remove unused ngtcp2_macro include in ngtcp2_qlog

Update ngtcp2_recv_crypto_data documentation

Use closure for scid and dcid generation

Remove unused parts of Buffer in example

Use char instead of string for \n in client.cc

This commit removes the include of ngtcp2_macro.h as it does not seem to
be used.

This commit contains a suggestion for improving ngtcp2_recv_crypto_data
documentation.

This commit suggests using a closure for the generation of ngtcp2_cids
in Client::init. The motivation for this is to reduce some minor code
duplication.

Remove unused parts of Buffer in example

A subsequent PR can further simplify `Buffer` as `begin` is now always equal to `head` and `size()` is always equal to `bufsize()`.

This commit updates write_transport_params in client.cc to use a
character '\n', instead of string "\n".

Remove unused ngtcp2_list_remove macro

Use OpenSSL return values for client_hello_cb

Remove unneeded assignments

This commit updates the client_hello_cb to use return values specified
in https://www.openssl.org/docs/man1.1.1/man3/
SSL_CTX_set_client_hello_cb.html.