Implement type-safe version of TOML ast

[chshersh]

So, currently TOML Value data type allows to have values of different type inside Array. I propose to solve this problem by introducing new TValue data type (or renaming current Value to UValue aka _untyped value) with GADT and DataKinds feature. Basically:

data TomlType = TBool | ...

data Value (t :: TomlType) where
    Bool :: Bool -> Value TBool
    ...

Basically, approach starting from this slide should be used:

• http://slides.com/fp-ctd/lecture-15#/8

I guess we should have UValue and typeCheck :: UValue -> Maybe Value function. Not sure though...

Also, several problems need to be solved. For example, this type also should have typed and untyped version but it's not obvious how to modify it... Let's discuss and see whether it makes sense!

data TOML = TOML
    { tomlPairs       :: HashMap Key     Value
    , tomlTables      :: HashMap TableId TOML