`push!!` fails for typed VarInfo when two `VarName`s have the same symbol but different type

[penelopeysm]

This means, for example, it is impossible to fill a typed VarInfo for models like this (instead you have to fill an untyped VarInfo and then only convert it to a typed VarInfo):

julia> using DynamicPPL, Distributions

julia> @model function f()
           x = (; a = undef, b = undef)
           x.a ~ Normal()
           x.b ~ Normal()
       end
f (generic function with 2 methods)

julia> DynamicPPL.init!!(f(), DynamicPPL.typed_varinfo(VarInfo()), InitFromPrior())
ERROR: MethodError: Cannot `convert` an object of type
  VarName{x,Accessors.PropertyLens{b}} to an object of type
  VarName{x,Accessors.PropertyLens{a}}
The function `convert` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  convert(::Type{T}, ::T) where T
   @ Base Base.jl:126

Stacktrace:
  [1] setindex!(h::Dict{VarName{:x, Accessors.PropertyLens{:a}}, Int64}, v0::Int64, key0::VarName{:x, Accessors.PropertyLens{:b}})
    @ Base ./dict.jl:346
  [2] push!(meta::DynamicPPL.Metadata{Dict{…}, Vector{…}, Vector{…}, Vector{…}}, vn::VarName{:x, Accessors.PropertyLens{…}}, r::Float64, dist::Normal{Float64})
    @ DynamicPPL ~/ppl/dppl/src/varinfo.jl:1734
  [3] push!!(vi::VarInfo{@NamedTuple{…}, DynamicPPL.AccumulatorTuple{…}}, vn::VarName{:x, Accessors.PropertyLens{…}}, r::Float64, dist::Normal{Float64})
[...]

An even smaller repro:

julia> vi = DynamicPPL.typed_varinfo(VarInfo());

julia> vi = push!!(vi, @varname(x.a), 1.0, Normal());

julia> vi = push!!(vi, @varname(x.b), 2.0, Normal())
ERROR: MethodError:
[...]

[penelopeysm]

I think this might mostly be a case of using BangBang.push!! somewhere instead of ordinary push!, but I'm not sure about potential implications on performance / type stability etc