[Bug #20649] Allow `nil` as 2nd argument of `assign_error`

	Fallback to the last token element in that case, for the backward
	compatibilities.

	[Bug #20517] Make a multibyte character one token at meta escape

	[Bug #20030] dispatch invalid escaped character without ignoring it

	[Bug #19788] Use the result of `tCOLON2` event

	---
	 parse.y                            | 16 ++++++++--------
	 test/ripper/test_parser_events.rb  | 17 +++++++++++++++++
	 test/ripper/test_scanner_events.rb |  5 +++++
	 3 files changed, 30 insertions(+), 8 deletions(-)

	Fix memory leak for incomplete lambdas

	[Bug #19836]

	The parser does not free the chain of `struct vtable`, which causes
	memory leaks.

	The following script reproduces this issue:

	```
	10.times do
	  100_000.times do
	    Ripper.parse("-> {")
	  end

	  puts `ps -o rss= -p #{$$}`
	end
	```
	---
	 parse.y                    | 24 ++++++++++++++----------
	 test/ripper/test_ripper.rb |  7 +++++++
	 2 files changed, 21 insertions(+), 10 deletions(-)

	Fix memory leak in parser for incomplete tokens

	[Bug #19835]

	The parser does not free the `tbl` of the `struct vtable` when there are
	leftover `lvtbl` in the parser. This causes a memory leak.

	The following script reproduces this issue:

	```
	10.times do
	  100_000.times do
	    Ripper.parse("class Foo")
	  end

	  puts `ps -o rss= -p #{$$}`
	end
	```
	---
	 parse.y                    | 42 ++++++++++++++++++++++++++++--------------
	 test/ripper/test_ripper.rb |  7 +++++++
	 2 files changed, 35 insertions(+), 14 deletions(-)

	[Bug #19399] Parsing invalid heredoc inside block parameter

	Although this is of course invalid as Ruby code, allow to just parse
	and tokenize.
	---
	 ext/ripper/lib/ripper/lexer.rb |  2 +-
	 test/ripper/test_lexer.rb      | 12 ++++++++++++
	 2 files changed, 13 insertions(+), 1 deletion(-)

`Ripper::Lexer#parse` re-parses the source code with syntax errors
when `raise_errors: false`.

Co-Authored-By: tompng <tomoyapenguin@gmail.com>