Make minidump-common::format structs writable through scroll.

[afranchuk]

As far as I can tell, there was no reason for these structs to not implement Pwrite. This is needed for rust-minidump/minidump-writer#8.

[luser]

Agreed. We simply didn't have any minidump-writing code when I first switched over to using scroll.

I wouldn't block on this, but it might be worth looking at the impact on code size/build time to decide whether it's worth using cargo features to conditionally enable the Pread / Pwrite derives? Presumably minidump-writer doesn't need Pread, and minidump-stackwalk doesn't need Pwrite.

[afranchuk]

On my system:

	release compile time	size
with Pread/Pwrite/SizeWith	7.32s	15MB
without	6.86s	12MB

Averaged over 10 runs.

[gabrielesvelto]

If crate size is an issue I suppose it's rather easy to add something like:

#[cfg_attr(feature = "write_support", derive(Pwrite)]

I don't know what crate sizes are considered acceptable these days. The increase in compilation time seems very marginal.

[luser]

Given @afranchuk's testing, I don't think it's necessary. If it becomes a problem in the future we can address it.

[gabrielesvelto]

Alright, merging this