add checked_{add,sub,mul} for BigInt
fix #5935 (rationalize returns curious/incorrect values)

Partially addresses #7501.
Add experimental ARM support to the README.
Merge pull request #8216 from JuliaLang/mn/rationalize

fix rationalize (#5935)
fix convert(Int64,::BigFloat) bug on 32-bit systems (closes #8318)
move Base.random.__init__ to zero-arg srand(); fixes #8313
fix rationalize test failure on 32-bit systems
Merge pull request #8321 from JuliaLang/mn/8318

fix a couple of 32-bit bugs
Merge pull request #8320 from davidssmith/srand

move Base.random.__init__ to zero-arg srand(); fixes #8313
Documentation for zero argument srand() #8320
fix #8311, spurious warning about importing Core.eval into Main
fix various doc typos in base.rst
Merge pull request #8325 from rfourquet/doc-typos

fix various doc typos in base.rst
Merge branch 'upstream-master'