rcutils_concat() should use an allocator

[sloretz]

rcutils_concat() currently uses malloc() internally instead of an allocator.

rcutils/src/concat.c

Line 40 in 9092a25

char * concat = (char *) malloc((lhs_len + rhs_len + del_len + 1) * sizeof(char));

[mikaelarguedas]

👍
Apparently the issue tracking that has been closed #4

[wjwwood]

Concat was removed in #102