New iread function.

The read function no longer works like it used to on an
interactive terminal because of the support for .. and .
syntax on a top-level expression.

The iread function is provided which uses a modified syntax
that doesn't support these operators on a top-level
expression. The parser thus doesn't look one token ahead,
and so iread can return immediately.

* eval.c (eval_init): Register iread intrinsic function.

* parser.c (prime_parser):  Only push back the recently seen
token when priming for a regular Lisp read. Handle
the prime_interactive method by preparing a SECRET_ESCAPE_I
token.
(lisp_parse_impl): New static function, formed from previous
lisp_parse. Takes a boolean argument indicating interactive
mode.
(prime_parser_post): New function.
(lisp_parse): Now a wrapper for lisp_parse_impl which
passes a nil to indicate noninteractive read.
(iread): New function.

* parser.h (enum prime_parser): New member, prime_interactive.
(scrub_scanner, iread, prime_parser_post): Declared.

* parser.l (prime_scanner): Handle the prime_interactive case
the same way as prime_lisp.
(scrub_scanner): New function.

* parser.y (SECRET_ESCAPE_I): New token type.
(i_expr): New nonterminal symbol. Like n_expr, but doesn't
support dot or dotdot operators, except in nested
subexpressions.
(spec): Handle SECRET_ESCAPE_I by way of i_expr.
(sym_helper): Before freeing the token lexeme, call
scrub_scanner. If the token is registered as the scanner's
most recently seen token, the scanner must forget that
registration, because it is no longer valid.
(parse): Call prime_parser_post.

* txr.1: Documented iread.

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