Deduplicate strucchange code in BFAST

[GreatEmerald]

bfast01 (classify?) is using some code taken from strucchange; try to deduplicate it by integrating the changes into strucchangeRcpp.

[GreatEmerald]

The functions in question are bfast01.confint and breakpoints.confint. To do: make tests in the examples section for both, then see if breakpoints.confint can be generalised to cover both cases. E.g. run breakpoints(breaks=1) and see if it's estimated the same way with bfast01.confint.