[struct_pack] support fast_varint encode, allow encode int as varint by config

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Why

1. varint code is slow when serialization/deserialization.
2. it's inconvenient to use struct_pack::var_int**_t, user may just want to use normal int type with varint encode.

What is changing

1. add a new encode named fast_varint. It 's very fast when serialization/deserialization. It can be used if your int field is likely to be zero or it max value in stuct is small likely.
2. allow user encode normal int as varint to simply usage.

For encoding detail, see document[TODO]

Example

Here is a struct:

using namespace struct_pack;
struct example0 {
  int32_t a;
  int64_t b;
  uint32_t c;
  uint64_t d;
};

you can add config to change coding:

struct example1 {
  int32_t a;
  int64_t b;
  uint32_t c;
  uint64_t d;
  static constexpr auto struct_pack_config =
      USE_FAST_VARINT | ENCODING_WITH_VARINT | DISABLE_ALL_META_INFO;
};

you can also change encoding config by ADL:

struct example2 {
  int32_t a;
  int64_t b;
  uint32_t c;
  uint64_t d;
};

constexpr auto set_sp_config(example2*) {
  return USE_FAST_VARINT | ENCODING_WITH_VARINT | DISABLE_ALL_META_INFO;
}

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I'll add document later.