`zend_uchar` suggests that the value is an ASCII character, but here,
it's about very small integers. This is misleading, so let's use a
C99 integer instead.
On all architectures currently supported by PHP, `zend_uchar` and
`uint8_t` are identical. This change is only about code readability.
This adds support for internal enums with the same basic approach
as userland enums. Enum values are stored as CONSTANT_AST and
objects created during constant updating at runtime. This means
that we need to use mutable_data for internal enums.
This just adds basic support and APIs, it does not include the
stubs integration from #7212.
Closes GH-7302.
