Merge branch 'feature/system-calls' into 'master'

.gitignore

Makefile.build

examples/Makefile

examples/echo.c

examples/exit

examples/exit.c

examples/touch

examples/touch.c

lib/kernel/stdio.h

threads/thread.h

userprog/file_descriptors_map.c

userprog/populate_stack.c

userprog/syscall.c

userprog/syscall_close.c

userprog/syscall_create.c

userprog/syscall_exec.c

userprog/syscall_exit.c

userprog/syscall_filesize.c

userprog/syscall_halt.c

userprog/syscall_open.c