Add module for sentinel handling

Closes #37 (closed)

The new module provides the following functions and subroutines

value = sentinel_i1()
value = sentinel_i2()
value = sentinel_i4()
value = sentinel_i8()
value = sentinel_sp()
value = sentinel_dp()
value = sentinel_spc()
value = sentinel_dpc()
value = sentinel_char()
value = get_sentinel(mold)
call set_sentinel(value)
state = check_sentinel(value)