Use size_t instead of unsigned in small_mapt

[danpoe]

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[danpoe]

It is guaranteed by the preconditions of the method and the static assertions that index_fieldt is large enough to hold v << shift.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 5b5180d).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/108510425