Implement RelTypes module for user- and system-defined (de)serialization hooks

[bachdavi]

This PR adds a RelTypes module that exports two methods: JuliaType and from_rel. These can be used to register type conversions from Rel ValueTypes to native Julia types. Currently only from Rel and not to Rel is supported. Conversion does not happen eagerly but rather on read-time. Currently, only show_result will show the converted types.

To register a conversion, we have to create an overload of the JuliaType method. It takes three parameters:

• A Val containing the markers of the value type, such as (:MyInt,), or (:MyType, :foo, :bar). The first element is always the Rel name of the value type.
• A type describing the field types this ValueType has, such as Tuple{Int64,Int64} in the case of value type MyInt = Int, Int. It's either a single type or a tuple type.
• [Optional] A vector of parameters. A parameter is a non-symbol constant type in the signature. Such as value type MyInt = #(1), Int, Int.

Here is an example:

value type Point = Int, Int

struct Point
    x::Int64
    y::Int64
end
JuliaType(::Val{(:Point,)}, Tuple{Int64,Int64} = Point
from_rel(::Type{Point}, t) = Point(t...)

There are a few things left:

• Tests
• Finish system-defined ValueTypes.
• Fix hard-coded Column creation in results.jl
• Properly convert specialized types and values as well, currently, we only convert the types

[bachdavi]

@NHDaly and @quinnj Adding you as reviewers :)
It's heavily inspired by ArrowTypes ;)