Declare level type for default_metafmt

[omus]

While investigating invenia/Intervals.jl#144 I noticed these invalidations:

 inserting isless(a, b::Intervals.Endpoint{T, Intervals.Direction{:Left}(), B} where B where T) in Intervals at /Users/omus/.julia/dev/Intervals/src/endpoint.jl:155 invalidated:
   mt_backedges: 1: signature Tuple{typeof(isless), Base.CoreLogging.LogLevel, Any} triggered MethodInstance for <(::Base.CoreLogging.LogLevel, ::Any) (2 children)

 inserting isless(a::Intervals.Endpoint{T, Intervals.Direction{:Right}(), B} where B where T, b) in Intervals at /Users/omus/.julia/dev/Intervals/src/endpoint.jl:164 invalidated:
   mt_backedges: 1: signature Tuple{typeof(isless), Any, Base.CoreLogging.LogLevel} triggered MethodInstance for <(::Any, ::Base.CoreLogging.LogLevel) (3 children)
   41 mt_cache

These Any comparisons are created from Logging.default_metafmt which uses @nospecialize on all arguments. Using a type declaration removes these sources of invalidations. The other changes are minor refactoring to make the code clearer.