Elixir library to parse and generate RFC 8941 Structured Field Values for HTTP.

HTTP headers often need to carry complex structures such as lists of values.
[RFC 8941](https://tools.ietf.org/html/rfc8941) specifies a standard format
for these fields independent of the RFCs that define the headers.

## Usage

```elixir
iex> HttpStructuredField.parse("42")
{:ok, {:integer, 42}}

iex> HttpStructuredField.parse("4.5")
{:ok, {:decimal, 4.5}}

iex> HttpStructuredField.parse("?1")
{:ok, {:boolean, true}}

iex> HttpStructuredField.parse(~S("hello world"))
{:ok, {:string, "hello world"}}

iex> HttpStructuredField.parse("foo123/456")
{:ok, {:token, "foo123/456"}}

iex> HttpStructuredField.parse(":cHJldGVuZCB0aGlzIGlzIGJpbmFyeSBjb250ZW50Lg==:")
{:ok, {:binary, "pretend this is binary content."}}

iex> HttpStructuredField.parse("1; abc; b=?0")
{:ok, {:integer, 1, [{"abc", {:boolean, true}}, {"b", {:boolean, false}}]}}

iex> HttpStructuredField.parse("foo, bar")
{:ok, [{:token, "foo"}, {:token, "bar"}]}

iex> HttpStructuredField.parse("a=(1 2), b=3, c=4;aa=bb, d=(5 6);valid", type: :dict)
{:ok, [
  {"a", {:inner_list, [integer: 1, integer: 2]}},
  {"b", {:integer, 3}},
  {"c", {:integer, 4, [{"aa", {:token, "bb"}}]}},
  {"d", {:inner_list, [integer: 5, integer: 6], [{"valid", {:boolean, true}}]}}
]}
```

The parser uses [NimbleParsec](https://hex.pm/packages/nimble_parsec), so it's
strict.

It handles the funky syntax of parameters, nested lists, and dictionaries. You can
run it on any input and it will return a tagged tuple for a simple value
or an Elixir list for a list of values. If there are parameters, then the tuple
will have three elements, with the third being a list. Inner List types are
tagged tuples, as we need some place to put the parameters.

Parmeters and dictionary members are represented as lists of tuples where the
name is the first tuple element.

Dictionary types are unfortunately incompatible with lists, so you have to tell
the parser what to expect by adding the `type: :dict` option.

## Installation

If [available in Hex](https://hex.pm/docs/publish), the package can be installed
by adding `http_structured_field` to your list of dependencies in `mix.exs`:

```elixir
def deps do
  [
    {:http_structured_field, "~> 0.1.0"}
  ]
end
```

Documentation can be generated with [ExDoc](https://github.com/elixir-lang/ex_doc)
and published on [HexDocs](https://hexdocs.pm). Once published, the docs can
be found at [https://hexdocs.pm/http_structured_field](https://hexdocs.pm/http_structured_field).