bignum.c: rb_int_parse_cstr handle `0` strings

	[Bug #19390]

	We shouldn't check the string length when skipping zeros, as the
	string might only contains zero characters, resulting in an empty string.
	---
	 bignum.c               | 1 -
	 test/ruby/test_time.rb | 4 ++++
	 2 files changed, 4 insertions(+), 1 deletion(-)

	Add missing assertion

	---
	 test/ruby/test_time.rb | 3 +++
	 1 file changed, 3 insertions(+)

	[Bug #19296] Precheck bits of time components

	---
	 test/ruby/test_time.rb | 15 +++++++++++++++
	 time.c                 | 14 ++++++++------
	 2 files changed, 23 insertions(+), 6 deletions(-)

	[Bug #19292] Re-initialize tm when wday or yday is not set

	---
	 test/ruby/test_time.rb |  3 ++-
	 time.c                 | 31 ++++++++++++++++++++++---------
	 2 files changed, 24 insertions(+), 10 deletions(-)

* 4-digits or more is required as year
* Minutes and seconds parts are not ommittable

Which limits the precision of subsecond. Defaulted to 9, that
means nanosecond.

`Time.new` now parses strings such as the result of `Time#inspect`
and restricted ISO-8601 formats.

Add a new macro BASE_SLOT_SIZE that determines the slot size.

For Variable Width Allocation (compiled with USE_RVARGC=1), all slot
sizes are powers-of-2 multiples of BASE_SLOT_SIZE.

For USE_RVARGC=0, BASE_SLOT_SIZE is set to sizeof(RVALUE).