Make Message data type more flexible

[chshersh]

Required for #97.

Current structure of the Message data type:

data Message = Message
    { messageSeverity :: !Severity
    , messageStack    :: !CallStack
    , messageText     :: !Text
    }

log :: WithLog env Message m => Severity -> Text -> m ()

Proposed change:

data Msg sev = Msg
    { msgSeverity :: !sev
    , msgStack    :: !CallStack
    , msgText     :: !Text
    }

type Message = Msg Severity

log :: WithLog env (Msg sev) m => sev -> Text -> m ()

Few explanations:

1. Message data type now stores polymorphic severities, so user are able to use their own severities.
2. Message data type is renamed to Msg and Message is converted to the type Message alias for better backwards compatibility.