chore(src): format with cargo-fmt

feat(src): add meaningful message to panic in Calendar::days_in_month

feat(src): add calendar freezing possibility

feat(src): add setter methods for Calendar

Allows to change the current date of
already existing calendar.

feat(src): implement Calendar::from_ticks for arbitrary year, add Calendar::to_ticks

So far, Calendar::from_ticks could not correctly parse
other month than the base month, this commit fixes that.

Calendar::to_ticks may convert any Calendar date to seconds passed from
the base year. The base year is remembered in Calendar instance.

chore(src): move source to source/ folder

feat(src): add app state with calendar