Issue with BigInt <: Signed

[omus]

Changing BigInt to be a subtypes of Signed can cause issues when promotion is skipped. For example:

Julia 0.6.0

julia> mod(BigInt(6), UInt128(5))  # Calls `mod(::Real, ::Real)`
1

Julia 0.7.0-DEV.2280 (d3afdfd)

julia> mod(BigInt(6), UInt128(5))  # Calls `mod(::Signed, ::Unsigned)`
ERROR: MethodError: Cannot `convert` an object of type BigInt to an object of type Unsigned
This may have arisen from a call to the constructor Unsigned(...),
since type constructors fall back to convert methods.
Stacktrace:
 [1] unsigned(::BigInt) at ./int.jl:148
 [2] mod(::BigInt, ::UInt128) at ./int.jl:210