Type assertions in converting offsets to Ints

[jishnub]

The inner OffsetArray constructor requires the offsets to be Ints, and #188 allowed other Integer types to be converted to Ints. This PR adds a type assertion in the outer constructor to avoid a stack overflow if the conversion doesn't work as expected.

[codecov]

Codecov Report

Merging #191 (6577f3f) into master (e65fd01) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #191   +/-   ##
=======================================
  Coverage   99.28%   99.28%           
=======================================
  Files           5        5           
  Lines         280      280           
=======================================
  Hits          278      278           
  Misses          2        2           

Impacted Files	Coverage Δ
src/OffsetArrays.jl	98.93% <100.00%> (ø)

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