added toml_kind_* to inspect item types

[jakergrossman]

Rationale

While using this for something I wanted to allow multiple types for an item, and more to the point, check that an item was one of a set of types. I found myself using an ugly idiom
where I was blindly trying toml_*_in and checking the result to know when it was the right type.

With access to the innards of the opaque types, this set of fairly straightforward functions allow a more elegant idiom:

# example.toml
name = "example"
major_version = 0

// prog.c
#include <toml.h>

int main()
{
    // ... omitted for brevity ...
    toml_table_t* conf = toml_parse_file("example.toml", errbuf, sizeof(errbuf));
    // ... omitted for brevity ...

    printf("%c\n", toml_kind_in(conf, "name")); // 's'
    printf("%c\n", toml_kind_in(conf, "major_version")); // 'i'

    printf("%d\n", toml_kind_one_of_in(conf, "name", "sid"); // 1
    printf("%d\n", toml_kind_one_of_in(conf, "name", "id"); // 0
    
}

The Solution

Adds a new toml_kind_t type:

enum toml_kind_t {
  TOML_KIND_UNKNOWN = 'u',
  TOML_KIND_STRING = 's',
  TOML_KIND_INT    = 'i',
  TOML_KIND_BOOL   = 'b',
  TOML_KIND_DOUBLE = 'd',
  TOML_KIND_TABLE  = 'B', // changed from previous version
  TOML_KIND_ARRAY  = 'a',
  TOML_KIND_DATE   = 'D',
  TOML_KIND_TIME   = 't',
  TOML_KIND_TIMESTAMP = 'T',
  TOML_KIND_MIXED = 'm',
  TOML_KIND_VALUE = 'v',
};

Unfortunately TOML_KIND_TABLE had to be disambiguated with TOML_KIND_TIME, and now functions that used to return 't' for table (like toml_array_kind()) now return 'B'. I am not sure if this is the best solution.