Squashed 'sequence/' changes from efeb0fc..cdae072

cdae072 removed many warnings
0226a4c more warnings enabled; fix tests
22705b2 change name of IO functions (keep compat)
REVERT: efeb0fc merge from master; version 0.5.4
REVERT: 2691bee version 0.5.3
REVERT: a373739 merge from master
REVERT: 0d721a6 release 0.5.2
REVERT: a50f307 merge from master
REVERT: e5625c6 merge from master: version 0.5.1
REVERT: 2ae771f oasis files

git-subtree-dir: sequence
git-subtree-split: cdae072e5e48d958d903e7aadfb23a022034707a
Merge commit 'c907b3d89131600fe153b0d7ad37e08d64c108f6' from sequence
updated sequence.mli