[parser] Decompose rule barestmt

[happy-barney]

• Concern separation

By extracting complex rules into standalone nonterminals code will become
easier to read and maintain.

Nonterminals can be easily ordered (e.g.: alphabetically) as well as reordered.

Leveraging bison's implicit rule such structure contain less noise.

Goal is to make barestmt to look like (simplificed):

barestmt
	:	PLUGSTMT
	|	bare_statement_class_declaration
	|	bare_statement_class_definition
	|	bare_statement_given
	|	bare_statement_null
	|	bare_statement_try
	|	bare_statement_yadayada
	;

• Nonterminal readability

Extracted nonterminals will contain single sequence so for readability purpose
tabular format will be used:

bare_statement_class_declaration
	:	KW_CLASS
		BAREWORD[version]
		BAREWORD[package]
		PERLY_SEMICOLON
		{
			... action
		}
	;

Single token per line limits number of required eye movements as well as
it provides indentation separation of terminals/nonterminals and actions:

• 1st tab indent: grammar operator
• 2nd tab indent: terminal/nonterminal
• 3rd+ tab indent: action code

• This set of changes does not require a perldelta entry.

[happy-barney]

I'm pushing changes from my GLR parser work, which are compatible with LALR(1) grammar,
so can rebase quickly.

[jkeenan]

I'm pushing changes from my GLR parser work, which are compatible with LALR(1) grammar, so can rebase quickly.

Please fix the porting test failures, get make test_porting && make test_harness to pass, then re-push. Thanks.

[happy-barney]

@jkeenan done - during rebase I forgot to run regen perly at some point and relied on test result from before rebase