Remove default MODEL from posix.mak

DMD compiles and work fine in 64 bit, so it makes no sense to set the
default MODEL to 32 instead of just let the compiler pick the native
model.
Allow string literals to be cast to mutable

It is legal in constfolding, and it's already impossible
to modify a string literal in CTFE.
CTFE: Fix comparison of string enums

String enum compares was failing to compile because it wasn't
checking the base type, so wasn't recognizing it as a string.
Merge pull request #1782 from leandro-lucarella-sociomantic/no-default-model

[trivial] Remove default MODEL from posix.mak
CTFE: Don't declare tuples wrappers as variables

Only the members of the tuples are variables. The tuple itself
doesn't need storage.
CTFE: Don't declare local enums as variables

They don't go on the stack (they already get a slot in the
global constants table when they get used)
Merge pull request #1807 from donc/ctfeEnumsAintVars

CTFE: Enums and tuple wrappers are not variables
Merge pull request #1804 from donc/string_enum

Fix treatment of string enums in CTFE