Improve parser support

[cpfiffer]

Please see #143, and #68 which this will eventually address.

This adds support for a typed JSON from arbitrary structs, such as

# Simple flat structure where each field is a primitive type
struct SimpleSingleton
    singleton_value::Int
end

typed_json_schema(SimpleSingleton)

Dict{Any, Any} with 1 entry:
  :singleton_value => "integer"

# Test a struct that contains another struct.
struct Nested
    inside_element::SimpleSingleton
end

typed_json_schema(Nested)

Dict{Any, Any} with 1 entry:
  :inside_element => Dict{Any, Any}("singleton_value" => "integer")

# Test a struct with a vector of primitives
struct ABunchOfVectors
    strings::Vector{String}
    ints::Vector{Int}
    floats::Vector{Float64}
    nested_vector::Vector{Nested}
end

typed_json_schema(ABunchOfVectors)

Dict{Any, Any} with 4 entries:
  :strings       => "string[]"
  :ints          => "integer[]"
  :nested_vector => Dict("list[Object]"=>"{\"inside_element\":{\"singleton_value\":\"integer\"}}")
  :floats        => "number[]"

It isn't really integrated with the rest of the ecosystem yet, I may need help from @svilupp on that one.