[parser] Decompose 'barestmt'

* 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

perly.y

@@ -343,9 +343,15 @@ labfullstmt:	LABEL barestmt
 			}
 	;
 
-/* A bare statement, lacking label and other aspects of state op */
-barestmt:	PLUGSTMT
-			{ $$ = $PLUGSTMT; }
+/* A bare statement, lacking label and other aspects of state op
+ *
+ * Maintain rule as simple alternatives of single nonterminals,
+ * leveraging bison's default action: $$ = $1.
+ *
+ * Try to keep nonterminals alphabetically sorted.
+ */
+barestmt
+	:	PLUGSTMT
 	|	KW_FORMAT startformsub formname formblock
 			{
 			  CV *fmtcv = PL_compcv;