L13: add code examples

Lectures/13/min_idx/Makefile

Lectures/13/min_idx/main.c

Lectures/13/roots/.gitignore

Lectures/13/roots/Makefile

Lectures/13/roots/roots.c

Lectures/13/roots/roots.h

Lectures/13/roots/roots_test.c

Lectures/13/sum_first_n_modular/.gitignore

Lectures/13/sum_first_n_modular/Makefile

Lectures/13/sum_first_n_modular/main.c

Lectures/13/sum_first_n_modular/min.c

Lectures/13/sum_first_n_modular/min.h

Lectures/13/sum_first_n_modular/min_test.c

Lectures/13/sum_first_n_modular/sum_first_n.c

Lectures/13/sum_first_n_modular/sum_first_n.h

Lectures/13/sum_first_n_modular/sum_first_n_test.c