Support external objects

[IsaacWoods]

AML provides a mechanism to declare that an object exists, but is not defined in this table. This is generally used for objects that are defined in an SSDT, so parsing can continue until they're parsed. However, it's mainly useful for ASL compilers and decompilers - I'm not sure we ever need to act on it (but shouldn't crash if we get one).

• Parse DefExternal
• Work out if we need to record them

[IsaacWoods]

A good first step here would be to parse DefExternal correctly, but do nothing with the result. This should be an easy first issue.