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

feat(src): add calendar freezing possibility

feat(src): replace DisplayView with hide, show on clock parts

The clock is meant to show only clock, so introducing
DisplayView trait was an overkill, a simpler
solution has been implemented instead.

Previous solution did not allow for showing and hiding individual parts,
hiding parts may be useful when setting time, to blink the current part
being set.

feat(src): add setter methods for Calendar

Allows to change the current date of
already existing calendar.

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