avoid abstract types

[johnnychen94]

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

This introduces unnecessary runtime overheads:

using BenchmarkTools, OpenCV

A_abs = rand(5,4,3);
A = OpenCV.Mat(A_abs);

@btime size($A); # 15.539 ns (1 allocation: 32 bytes)
@btime size($A_abs); # 1.430 ns (0 allocations: 0 bytes)

Looks like Mat and Vec should be updated accordingly. There might be some other cases, though.