Built-in BNF items for pgen

The built-in BNF items for ''pgen'' are:

INT: integers
REAL: real numbers
ID: C-style identifiers
STRING: strings
expr: C-style expressions of any type
int_expr: integer expressions
bool_expr: Boolean expressions

If any of the expression constructs are used, then the user must also define the BNF item expr_id, which is used to parse the identifiers that can be part of an expression.

Type	Value
`INT`	`int`
`REAL`	`double`
`ID`	`char *`
`STRING`	`char *`
`expr`	`Expr *`
`int_expr`	`Expr *`
`bool_expr`	`Expr *`

The char * returned for a STRING includes the two double-quotes that delimit the string.