Integer overflow in specialized _solve methods

As observed in #583 there's some weird errors with small-sized solves of integer matrices. For example:

julia> m = 2
       a = SMatrix{m,m}(rand(Int,m,m))
       b = SVector{m}(rand(Int,m))
2-element SArray{Tuple{2},Int64,1,2} with indices SOneTo(2):
  8315397486252833078
 -1243568792894157487

julia> Matrix(a) \ Vector(b)
2-element Array{Float64,1}:
 -1.0427738899729126
  0.5986498869255282

julia> a \ b
2-element SArray{Tuple{2},Float64,1,2} with indices SOneTo(2):
 4.043446204424333e-20
 8.620209721520739e-20

The underlying cause seems to be integer overflow in some specialized _solve functions. For example due using integer arithmetic here: https://github.com/JuliaArrays/StaticArrays.jl/blob/973a7092e1b92a95b69f4611e9da2c39707029bc/src/solve.jl#L10

CC @judober

Start in src/solve.jl at the specialized _solve code around the referenced line and reproduce the small integer-matrix example from the issue. Trace the integer arithmetic in the specialized solve methods and compare it with the dense solve result; done means integer inputs no longer overflow and produce the correct solution.