Fix type stability of Base.RefValue fields

[odow]

I was trying to figure out why #241 was needed. Benchmarking showed some allocations that I didn't expect. Then I found that unsafe_convert was typed as ::Any!

julia> import SCIP

julia> import MathOptInterface as MOI

julia> model = SCIP.Optimizer();

julia> @time SCIP.SCIPgetSols(model)
  0.000005 seconds (2 allocations: 32 bytes)
Ptr{Ptr{Nothing}} @0x0000000000000000

julia> @time SCIP.SCIPgetNSols(model)
  0.000007 seconds (1 allocation: 16 bytes)
0

julia> @code_warntype SCIP.SCIPgetNSols(model)
MethodInstance for SCIP.LibSCIP.SCIPgetNSols(::SCIP.Optimizer)
  from SCIPgetNSols(scip) @ SCIP.LibSCIP ~/.julia/packages/SCIP/9Wf6c/src/LibSCIP.jl:18286
Arguments
  #self#::Core.Const(SCIP.LibSCIP.SCIPgetNSols)
  scip::SCIP.Optimizer
Body::Int32
1 ─ %1 = Core.apply_type(SCIP.LibSCIP.Ptr, SCIP.LibSCIP.SCIP)::Core.Const(Ptr{Nothing})
│   %2 = Base.cconvert(%1, scip)::SCIP.Optimizer
│   %3 = Core.apply_type(SCIP.LibSCIP.Ptr, SCIP.LibSCIP.SCIP)::Core.Const(Ptr{Nothing})
│   %4 = Base.unsafe_convert(%3, %2)::Any
│   %5 = $(Expr(:foreigncall, :(Core.tuple(:SCIPgetNSols, SCIP.LibSCIP.libscip)), Int32, svec(Ptr{Nothing}), 0, :(:ccall), :(%4), :(%2)))::Int32
└──      return %5

It's because of this classic gag that Base.Ref{T} is not a concrete value:

julia> x = Ref{Cint}(1)
Base.RefValue{Int32}(1)

julia> typeof(x)
Base.RefValue{Int32}

julia> typeof(x) <: Ref{Cint}
true

The fix is a couple of uses of Base.RefValue:

julia> import SCIP

julia> import MathOptInterface as MOI

julia> model = SCIP.Optimizer();

julia> @time SCIP.SCIPgetSols(model)
  0.000008 seconds (1 allocation: 16 bytes)
Ptr{Ptr{Nothing}} @0x0000000000000000

julia> @time SCIP.SCIPgetNSols(model)
  0.000005 seconds
0

julia> @code_warntype SCIP.SCIPgetNSols(model)
MethodInstance for SCIP.LibSCIP.SCIPgetNSols(::SCIP.Optimizer)
  from SCIPgetNSols(scip) @ SCIP.LibSCIP ~/.julia/dev/SCIP/src/LibSCIP.jl:18291
Arguments
  #self#::Core.Const(SCIP.LibSCIP.SCIPgetNSols)
  scip::SCIP.Optimizer
Body::Int32
1 ─ %1 = Core.apply_type(SCIP.LibSCIP.Ptr, SCIP.LibSCIP.SCIP)::Core.Const(Ptr{Nothing})
│   %2 = Base.cconvert(%1, scip)::SCIP.Optimizer
│   %3 = Core.apply_type(SCIP.LibSCIP.Ptr, SCIP.LibSCIP.SCIP)::Core.Const(Ptr{Nothing})
│   %4 = Base.unsafe_convert(%3, %2)::Ptr{Nothing}
│   %5 = $(Expr(:foreigncall, :(Core.tuple(:SCIPgetNSols, SCIP.LibSCIP.libscip)), Int32, svec(Ptr{Nothing}), 0, :(:ccall), :(%4), :(%2)))::Int32
└──      return %5

[matbesancon]

Good catch thanks! Merged and released