improve strcat, strsubst, part performance

improve strcat, strsubst, part performance

//strcat
a = ones(1,300000);
b = string(a);
tic();strcat(b);toc()

//strsubst
A = string(rand(100, 100));
tic;B = strcat(A);toc
tic;strsubst(B, "3", "X");toc
tic;strsubst(B, "30", "X");toc
tic;strsubst(B, "3", "XYZ");toc

//part
c = 1e5;
str = strcat(string(int(rand(1, c) * 10)));
idx = int(rand(1, c) * c + 1);
tic;part(str, idx);toc

Change-Id: I8f2635cf3ce2e3ab257690935ddf0961b403ea40