missing floatmin, floatmax, issubnormal

One would expect the following to be defined for any floating-point type:
```jl
julia> x = dec"10.0"
10.0

julia> floatmin(x)
ERROR: MethodError: no method matching floatmin(::Type{Decimal})

julia> floatmax(x)
ERROR: MethodError: no method matching floatmax(::Type{Decimal})

julia> issubnormal(x) # expected result: false
ERROR: MethodError: no method matching issubnormal(::Decimal)
```
You clearly have a non-trivial `floatmin` and `floatmax` because raising to a large power gives an overflow error
```jl
julia> (dec"10")^1000000000
ERROR: OverflowError: Exponent limit (999999) exceeded: Decimal(0, 1000000000000000000000000000000000000000000000000000000, 1048522)
```
and raising a number `< 1` to a large power gives the wrong result:
```jl
julia> (dec"0.1")^1000000000
0E-1000026
```