tslib flake8 cleanup, missing declarations
replace `isinstance` checks with the faster C versions
change cpdef-->cdef for internal field-getter methods
remove get_named_field in favor of faster dict lookup (yah yah I'll show asv results in a bit)

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
