This commit:
* Introduces `_` as a prefix for all non-user entities we emit in
decompiled code.
Also, some names have been changed to be more concise.
Specifically, the following entities have changed:
`_ENUM_UNDERLYING`, `_ABI`, `_REG`, `_padding_at_`,
`_artificial_struct_`, `_artificial_wrapper_`, `_stack`,
`_break_from_loop_`, `_var_`, `_stack_arguments`,
`_artificial_struct_returned_`, `_enum_max_value_`.
* Introduce _PACKED for `__attribute__((packed))`.
* `EnumEntry` name: drop the `EnumType` name prefix.