avoid fields with abstract types (#100)

https://docs.julialang.org/en/v1/manual/performance-tips/#Avoid-fields-with-abstract-type

The Buffer contains a field which is of type Array{T} where T. This is an abstract type, and to make it concrete, we need to specify the dimension of the array (2).

src/PortAudio.jl

@@ -379,7 +379,7 @@ Use [`PortAudio.write_buffer`](@ref) to write data to PortAudio, and [`PortAudio
 struct Buffer{Sample}
     stream_lock::ReentrantLock
     pointer_to::Ptr{PaStream}
-    data::Array{Sample}
+    data::Array{Sample, 2}
     number_of_channels::Int
     frames_per_buffer::Int
     warn_xruns::Bool