src: use explicit casts to silence conversion warnings
Addresses ~30 of the warnings posted in https://github.com/nodejs/node/issues/26733#issuecomment-473894753. (I don't think there's a better solution than accepting the precision loss, since bigint is already an available option.)
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
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