optimize the memory layout of Module.Fn and Module.Var

`is_pub` added to `Fn` would cost us an additional 8
bytes of memory per function, which is a real bummer
since it's only 1 bit of information.

If we wanted to really remove this, I suspect we could
make this a function isPub() which looks at the AST of
the corresponding Decl and finds if the FnProto AST node
has the pub token. However I saw an easier approach -

The data of whether something is pub or not is actually
a property of a Decl anyway, not a function, so we can
look at moving the field into Decl. Indeed, doing this,
we see that Decl already has deletion_flag: bool which
is hiding in the padding bytes between the enum (1 byte)
and the following u32 field (generation). So if we put
the is_pub bool there, it actually will take up no
additional space, with 1 byte of padding remaining.

This was an easy reworking of the code since any
func.is_pub could be changed simply to func.owner_decl.is_pub.

I also modified `Var` to make the init value non-optional
and moved the optional bit to a has_init: bool field. This is worse from
the perspective of control flow and safety, however it makes
`@sizeOf(Var)` go from 32 bytes to 24 bytes. The more code we can fit
into memory at once, the more justified we are in using the compiler as
a long-running process that does incremental updates.

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