fix https://github.com/msoucy/dproto/issues/115

[timotheecour]

this should fix #115.

side question for @msoucy:

please see comment in diff:
should i use readVarint!(Unsigned!T2)() instead of src.readVarint() in static if(T == "sint32" || T == "sint64") block?
currently, it'll force to use ulong and then call fromZigZag().to!T2(); (ie convert to a int or long)
with the code in comment, it'll read into a uint or ulong, then call zigzag to convert to int or long.

[coveralls]

Changes Unknown when pulling 69776bd on timotheecour:fix_115_overflow into ** on msoucy:master**.

[timotheecour]

@msoucy I just added a unittest; it shows additional bugs for types:
"sint64" , "sint32"; however, these are pre-existing bugs so shouldn't block this PR, but I'm curious what you think would fix it?

[coveralls]

Changes Unknown when pulling c7209fd on timotheecour:fix_115_overflow into ** on msoucy:master**.

[coveralls]

Changes Unknown when pulling 909454f on timotheecour:fix_115_overflow into ** on msoucy:master**.

[timotheecour]

@msoucy
seems like the travis failures are ok: works on recent version of dmd,ldc; should we maybe remove old versions where it fails?

[msoucy]

I'm never sure which versions of dmd to with Travis... It's too bad that there isn't a way to specify "the versions that are available to the package managers for commonly used distros".

I'll make that update to .travis.yml, but I'm not sure that it'll help this. Versions of LDC that are accessible by supported releases (without adding other sources, of course) are 0.16.1 (Fedora 24, DMD frontend 2.067.1), 0.17.3 (Fedora 25, DMD frontend 2.068.2), and 1.1.1 (Fedora 26, DMD frontend 2.071.2). I can't find the versions of the DMD frontend supported by the Debian packages, though, only the GCC backend. It looks like the DMD frontend supported by its most recent version is 2.068.2, though.

Though I haven't been following this as strictly as I should have, I would prefer that dproto is usable on all compilers that are installable by default on Fedora and Debian.

[timotheecour]

any comments on this PR?
can we ignore the travis failures since https://travis-ci.org/msoucy/dproto shows git master is also failing for some targets?

[redstar]

PR #123 fixes all Travis failures. After merging this PR should be green, too.

[coveralls]

Changes Unknown when pulling c6a5641 on timotheecour:fix_115_overflow into ** on msoucy:master**.

[coveralls]

Changes Unknown when pulling 5bf0a18 on timotheecour:fix_115_overflow into ** on msoucy:master**.

[redstar]

@timotheecour Please have a look at #130. I think you can fix your PR with code from mine.

[timotheecour]

done, closing in favor of #131