Replace StateT with Reader

Fixes #49.

The pure core only reads the Global state. As a result, I don't need the State monad and can replace it with the Reader monad to clearly reflect this requirement in the type system as well.

This provides an interesting design space with the following three options:

No Transformers, just Thread/Passthrough the Global argument:

getTypeOnHover :: Global  -> Position -> Maybe Hover
getTypeOnHover gl pos = do
    token      <- findToken gl pos
    qname      <- tokenToQName gl token
    symbol     <- findSymbol gl qname
    symbolType  = getSymbolType gl symbol
    pure        $ Hover 
        { 
            range   = tokenToRange token, 
            content = FregeCodeBlock symbolType
        }

Pros: Easier to understand because no monad transformers are involved, e.g. no lift.
Cons: All intermediary functions need the additional Global parameter. This makes not only the high-level function getTypeOnHover more noisy but also all intermediary functions, which do not actually use the Global argument. In addition, nothing prevents the user to modify the Global between function calls (e.g. between the findToken and the tokenToQName step, which violates the assumption that the Global is an immutable Global data type.

`MaybeT Reader Global a`

The code can be found on the f-readerT branch.

In order to combine it with the Maybe Monad, I opted for the MaybeT transformer monad. The liftMaybe function is responsible to lift Maybe values into the MaybeT monad and allows to combine the various search functions, which return Maybe values:

https://github.com/tricktron/frege-lsp-server/blob/a7241931652560324f8d8a4d585f3219081ebe10/src/main/frege/ch/fhnw/thga/fregelanguageserver/hover/Hover.fr#L39-L40

https://github.com/tricktron/frege-lsp-server/blob/a7241931652560324f8d8a4d585f3219081ebe10/src/main/frege/ch/fhnw/thga/fregelanguageserver/hover/Hover.fr#L74-L84

Pros: Keeps the higher-level function getTypeOnHover simpler. The Global data type is immutable as default. You could still run a computation in a different environment using the local function. However, the user needs to actively do that.
Cons: The lower-level function become more complex because we now use monad transformers, which do the work, e.g. we need to use lift to jump around the monad stack.

`ReaderT Maybe Global a` a.k.a `Kleisli Maybe Global a`

The code can be found on the f-readerT-arrow branch.

This stack has the same pros and cons as the MaybeT Reader Global a approach. However, it has some subtle differences:

Kleisli m a b is an instance of the Arrow type class and the monad type class in Frege. This is a major difference to Haskell where theReaderT is not an Arrow instance. According to the Haskell Typeclassopedia an Arrow and a Monad is not the same but Monad and ArrowApply are the same.

I have never used the Arrow type class, so the following may be completely wrong and inaccurate:

Since Kleisli is not an instance of MonadTrans we cannot use the standard lift function, e.g:

findSymbol :: QName -> ReaderT Maybe Global Symbol
findSymbol qname = do
    global     <- ask
    lift       $ Global.find global qname -- fails because Kleisli Maybe is not an instance of MonadTrans

In Haskell this works because there is a MonadTrans instance of ReaderT. I did not manage to write a MonadTrans (ReaderT) instance in Frege. Instead I wrote another helper function liftKleisli:

https://github.com/tricktron/frege-lsp-server/blob/7f790e28a9a9af3f1d4e5ee3b884250d94fd1fd0/src/main/frege/ch/fhnw/thga/fregelanguageserver/hover/Hover.fr#L34-L35

The high-level getTypeOnHover function then looks like this:

https://github.com/tricktron/frege-lsp-server/blob/7f790e28a9a9af3f1d4e5ee3b884250d94fd1fd0/src/main/frege/ch/fhnw/thga/fregelanguageserver/hover/Hover.fr#L65-L75

Instead of lift I used the arr function to lift a function into Kleisli m. This has the advantage that I can keep the getSymbolType function even simpler without making it a Reader:

Using Arrow with Kleisli: https://github.com/tricktron/frege-lsp-server/blob/7f790e28a9a9af3f1d4e5ee3b884250d94fd1fd0/src/main/frege/ch/fhnw/thga/fregelanguageserver/hover/Hover.fr#L37-L38

Vs. Using Monad with MaybeT: https://github.com/tricktron/frege-lsp-server/blob/a7241931652560324f8d8a4d585f3219081ebe10/src/main/frege/ch/fhnw/thga/fregelanguageserver/hover/Hover.fr#L42-L45

tricktron / frege-lsp-server