Re: [quicwg/base-drafts] Make using static table and Huffman encoding in QPACK opt-in (#3622)

[Lars Eggert <[e-mail]>]

With the ARM toolchain for the Cortex M4, here is the object size (second column) of the various functions in `h3zero.c`, largest first:
```
000007a8 00001010 D h3zero_qpack_huffman_table
00000000 00000630 D qpack_static
00000eb8 000003e8 T h3zero_parse_data_stream
00000c70 00000248 T h3zero_parse_qpack_header_frame
00000814 00000228 T hzero_qpack_huffman_decode
00000638 00000170 D h3zero_qpack_index
00000a3c 00000164 T h3zero_parse_qpack_header_value
000001e4 00000100 T h3zero_qpack_int_decode
000004c4 000000d4 T h3zero_create_post_header_frame
00000ba0 000000d0 T h3zero_get_interesting_header_type
0000008c 000000b8 T h3zero_qpack_int_encode
00000598 000000b8 T h3zero_create_request_header_frame
00000700 000000a4 T h3zero_create_bad_method_header_frame
00000370 00000094 T h3zero_get_content_type_by_name
000002e4 0000008c T h3zero_get_method_by_name
00000444 00000080 T h3zero_encode_content_type
00000014 00000078 T h3zero_varint_decode
00000174 00000070 t h3zero_qpack_literal_plus_ref_encode
000007a4 00000070 T h3zero_delete_data_stream_state
00000650 00000060 T h3zero_create_response_header_frame
000006b0 00000050 T h3zero_create_not_found_header_frame
00000404 00000040 T h3zero_parse_status
00000144 00000030 t h3zero_qpack_code_encode
00000000 00000014 T h3zero_varint_skip
0000006c 00000007 r h3zero_default_setting_frame_val
00000634 00000004 D nb_h3zero_qpack_index
000017b8 00000004 D nb_h3zero_qpack_huffman_table
00000630 00000004 D h3zero_qpack_nb_static
00000068 00000004 R h3zero_default_setting_frame_size
000017bc 00000004 D h3zero_default_setting_frame
```
So the table alone is about 1K. That's not great, but I'm also not sure if `qpack_static` is the most compact representation of this data.

In terms of code size, there are some easy gains just by using minimally-sized integer types. For example, only changing `index` to `uint8_t` and `enum_as_int` to `uint16_t` in `st_h3zero_qpack_static_t` saves you 368 bytes. If you can get rid of `uint64_t` as much as possible, that would probaly save you 1-2K in size right there. 

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