Address MSVC 64-bit compile warnings.

[toojays]

MSVC 2015 emitted a handful of truncation errors about implicitly converting from a size_t length argument to a uint32 or long. These all look intentional or benign. This commit adds casts to silence the warnings.

The errors addressed are:

3>external\city.cc(170): error C2220: warning treated as error - no 'object' file generated
3>external\city.cc(170): warning C4267: 'initializing': conversion from 'size_t' to 'uint32', possible loss of data
3>external\city.cc(183): warning C4267: 'argument': conversion from 'size_t' to 'uint32', possible loss of data
3>external\city.cc(187): warning C4267: 'initializing': conversion from 'size_t' to 'uint32', possible loss of data
3>external\city.cc(202): warning C4267: 'initializing': conversion from 'size_t' to 'uint32', possible loss of data
3>external\city.cc(307): warning C4267: 'initializing': conversion from 'size_t' to 'uint32', possible loss of data
3>external\city.cc(425): warning C4267: 'initializing': conversion from 'size_t' to 'long', possible loss of data

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[toojays]

I'm now covered by a corporate agreement.

[googlebot]

CLAs look good, thanks!

[lord-ne]

This may have been superceded by the most recently merged pull request?