Improve type-stability in reconstruct

[jishnub]

On master

julia> @with_kw struct A
       a::Int
       b::String
       end
A

julia> a = A(1, "a")
A
  a: Int64 1
  b: String "a"

julia> @code_warntype A(a)
MethodInstance for A(::A)
  from A(pp::A; kws...) @ Main ~/Dropbox/JuliaPackages/Parameters.jl/src/Parameters.jl:571
Arguments
  #self#::Core.Const(A)
  pp::A
Body::Any
1 ─ %1 = Main.:(var"#A#5")::Core.Const(Main.var"#A#5")
│   %2 = Core.NamedTuple()::Core.Const(NamedTuple())
│   %3 = Base.pairs(%2)::Core.Const(Base.Pairs{Symbol, Union{}, Nothing, @NamedTuple{}}())
│   %4 = (%1)(%3, #self#, pp)::Any
└──      return %4

This PR uses a tuple of fields to construct the type, so we obtain

julia> @code_warntype A(a)
MethodInstance for A(::A)
  from A(pp::A; kws...) @ Main ~/Dropbox/JuliaPackages/Parameters.jl/src/Parameters.jl:571
Arguments
  #self#::Core.Const(A)
  pp::A
Body::A
1 ─ %1 = Main.:(var"#A#5")::Core.Const(Main.var"#A#5")
│   %2 = Core.NamedTuple()::Core.Const(NamedTuple())
│   %3 = Base.pairs(%2)::Core.Const(Base.Pairs{Symbol, Union{}, Nothing, @NamedTuple{}}())
│   %4 = (%1)(%3, #self#, pp)::A
└──      return %4

The return type is inferred in this case.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.75%. Comparing base (501b47b) to head (d80e84f).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #160   +/-   ##
=======================================
  Coverage   93.75%   93.75%           
=======================================
  Files           1        1           
  Lines         304      304           
=======================================
  Hits          285      285           
  Misses         19       19           

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