travis: pin flow to master
Internal_pervasives: add Sequence module
Internal_pervasives: remove Parse sub-module

Contained only a single function, which was used by only one function
in Track. Moved function within Track module.
Internal_pervasives: simplify and rename

We now `open Core.Std` explicitly everywhere, and `open
Biocaml_internal_utils` when needed. This is easier to maintain as we
don’t have to keep up with changes to Core. Also it is safer; Core may
override or disable certain functions, which we might accidentally have
not included in our our manual list of functions to include.