Replace uses of 0 with nullptr when applicable.

This change was generated with
 'clang-modernize -use-nullptr src/main/* src/main/jit/*'
Replace uses of 0 with nullptr in headers.

This change was generated with 'clang-modernize -use-nullptr'.
Add 'override' declarations where appropriate.

This change was generated by 'clang-modernize -add-override' and minor
manual edits.
Change a bunch of 'for' loops to use C++11 range based 'for' loops.

This change was partly generated with 'clang-modernize -loop-convert', but
also involved significant manual editing for readability purposes.
Merge pull request #3 from kmillar/nullptr

Nullptr