tests(src): add tests for calendar tick functions

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): format with fmt

feat(src): split main into multiple functions

fix(src): remove unnecessary import

chore(src): move source to source/ folder