Add debug information for arrows.

[kindaro]

Closes #44.

[kindaro]

Example output on Test.LexTree:

Test.LexTree (test/Test/LexTree.hs)
    check (ValueDecl, 120:1)
    checkBalance (ValueDecl, 135:1)
    checkBounds (ValueDecl, 96:1)
        f (ValueDecl, 98:5)
    checkHeight (ValueDecl, 123:1)
        go (ValueDecl, 125:5)
            h' (ValueDecl, 130:11)
    foldLexTreeM (ValueDecl, 114:1)
    lookupStack (ValueDecl, 148:1)
        f (ValueDecl, 150:5)
    range (ValueDecl, 145:1)
    spec (ValueDecl, 55:1)
        f (ValueDecl, 80:11)
check ---> checkBalance
check ---> checkBounds
check ---> checkHeight
checkBounds ---> f
checkBounds ---> foldLexTreeM
checkHeight ---> go
go ---> h'
lookupStack ---> f
spec ---> check
spec ---> checkBalance
spec ---> checkBounds
spec ---> checkHeight
spec ---> f
spec ---> lookupStack

[jonascarpay]

Looks good, thanks!