Enhance compatibility with StaticArrays.jl

[hyrodium]

It would be great if JSON.parse could support parsing directly into StaticArrays.jl static arrays. For example, it would be nice if the following worked:

julia> using JSON, StaticArrays

julia> JSON.parse("[1,2]", Vector{Int})
2-element Vector{Int64}:
 1
 2

julia> JSON.parse("[1,2]", SVector{2,Int})
ERROR: MethodError: Cannot `convert` an object of type UndefInitializer to an object of type Int64
The function `convert` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  convert(::Type{T}, ::T) where T<:Number
   @ Base number.jl:6
  convert(::Type{T}, ::Number) where T<:Number
   @ Base number.jl:7
  convert(::Type{T}, ::T) where T
   @ Base Base_compiler.jl:133
  ...

Stacktrace:
  [1] macro expansion
    @ ~/.julia/packages/StaticArraysCore/7xxEJ/src/StaticArraysCore.jl:88 [inlined]
  [2] convert_ntuple
    @ ~/.julia/packages/StaticArraysCore/7xxEJ/src/StaticArraysCore.jl:84 [inlined]
  [3] SVector{2, Int64}(x::Tuple{UndefInitializer, Int64})
    @ StaticArraysCore ~/.julia/packages/StaticArraysCore/7xxEJ/src/StaticArraysCore.jl:120
  [4] StaticArray
    @ ~/.julia/packages/StaticArrays/DsPgf/src/convert.jl:173 [inlined]
  [5] initialize
    @ ~/.julia/packages/StructUtils/CvpPP/src/StructUtils.jl:172 [inlined]
  [6] makearray(style::JSON.JSONReadStyle{JSON.Object{String, Any}, Nothing}, ::Type{SVector{2, Int64}}, source::JSON.LazyValue{String})
    @ StructUtils ~/.julia/packages/StructUtils/CvpPP/src/StructUtils.jl:873
  [7] make(style::JSON.JSONReadStyle{JSON.Object{String, Any}, Nothing}, T::Type, source::JSON.LazyValue{String})
    @ StructUtils ~/.julia/packages/StructUtils/CvpPP/src/StructUtils.jl:779
  [8] _parse
    @ ~/.julia/packages/JSON/0oqO1/src/parse.jl:193 [inlined]
  [9] parse(x::JSON.LazyValue{String}, ::Type{SVector{…}}; dicttype::Type{JSON.Object{…}}, null::Nothing, style::JSON.JSONReadStyle{JSON.Object{…}, Nothing})
    @ JSON ~/.julia/packages/JSON/0oqO1/src/parse.jl:189
 [10] parse
    @ ~/.julia/packages/JSON/0oqO1/src/parse.jl:189 [inlined]
 [11] parse(buf::String, ::Type{SVector{…}}; dicttype::Type{JSON.Object{…}}, null::Nothing, style::JSON.JSONReadStyle{JSON.Object{…}, Nothing}, kw::@Kwargs{})
    @ JSON ~/.julia/packages/JSON/0oqO1/src/parse.jl:181
 [12] parse(buf::String, ::Type{SVector{2, Int64}})
    @ JSON ~/.julia/packages/JSON/0oqO1/src/parse.jl:181
 [13] top-level scope
    @ REPL[3]:1
Some type information was truncated. Use `show(err)` to see complete types.

I found that defining the following method makes it work:

julia> StructUtils.makearray(style, T::Type{<:StaticVector}, source) = T([source[i][] for i in 1:length(T)]), StructUtils.applyeach(style, StructUtils.ArrayClosure(eltype(T)[], style), source)

julia> JSON.parse("[1,2]", SVector{2,Int})
2-element SVector{2, Int64} with indices SOneTo(2):
 1
 2

However, I’m not sure which function would be the appropriate extension point for this. It’s also unclear to me which package should provide this support — JSON.jl, StaticArrays.jl, or possibly even StructUtils.jl. In any case, this would likely need to be implemented as a package extension. Since StructUtils.makearray does not appear to be documented, I’m not sure whether it is considered internal API.