Function handle support

[matthijscox]

We don't error anymore on function handles since MAT v0.11, but let's remember to provide good read-write support at some point:

Current behavior:

julia> sin_f = matread("test/v7.3/function_handles.mat")["sin"]
Dict{String, Any} with 4 entries:
  "function_handle" => Dict{String, Any}("function"=>"sin", "file"=>"", "type"=>"simple")
  "sentinel"        => '@'
  "separator"       => '/'
  "matlabroot"      => "/opt/MATLAB/R2018b"

julia> anon = matread("test/v7.3/function_handles.mat")["anonymous"]
Dict{String, Any} with 4 entries:
  "function_handle" => Dict{String, Any}("function"=>"sf%0@(x)x", "workspace"=>MatlabOpaque("any"=>Any[MatlabOpaque(); Dict{String, Any}();;]), "file"=>"", "within_file_path"=>"__base_function", "ty…
  "sentinel"        => '@'
  "separator"       => '/'
  "matlabroot"      => "/opt/MATLAB/R2018b"

When we write these, you get a struct in MATLAB.

[foreverallama]

I think we can already load function handles already, but as a struct/dict. We could use another wrapper MatlabFunction for this, it's what SciPy (and matio) uses.

I can take this up along with write support.