`using script_verify_flags = uint32_t` allows implicit conversion to
and from int, so replace it with a class to have the compiler ensure we
use the correct type. Provide from_int and as_int to allow for explicit
conversions when desired.
Introduces the type `script_verify_flag_name` for the individual flag
name enumeration.
Previously the SCRIPT_VERIFY_* flags were specified as either uint32_t,
unsigned int, or unsigned. This converts them to a common type alias in
preparation for changing the underlying type.
