Implicit enum conversions are a stupid PITA

[Walter Bright]

Adam D. Ruppe wrote:
> The argument most often brought up for keeping octal *at all* is unix
> filesystem permissions. They are only ever as big as four digits (AFAIK).

There is one other reason: converting C code to D code. This should follow the 
principle of "if it doesn't give a compiler error, it should produce the same 
result". Translating:

    0177

in C to:

    0177

in D will silently produce a very different result (should such a number be 
decimal).