Issue #143: full-line comments not allowed within transactions (blais/beancount)

[Stefano Zacchiroli]

New issue 143: full-line comments not allowed within transactions
https://bitbucket.org/blais/beancount/issues/143/full-line-comments-not-allowed-within

Stefano Zacchiroli:

According to my reading of the syntax reference, comment should be allowed everywhere in a beancount file, no matter if they are in-line comments or entire-line comments.

According to bean-check this is not the case though. While the following passes bean-check just fine:
```
#!beancount

2014-04-23 * "Flight to Berlin" #berlin-trip-2014
foo: "bar"
Expenses:Flights -1230.27 USD
foo: "baz" ; in-line comment 1
Liabilities:CreditCard ; in-line comment 2
foo: "qux"
```
the following (note the addition of "; full-line comment 3") does not:
```
#!beancount

2014-04-23 * "Flight to Berlin" #berlin-trip-2014
foo: "bar"
Expenses:Flights -1230.27 USD
; full-line comment 3
foo: "baz" ; comment 1
Liabilities:CreditCard ; comment 2
foo: "qux"
```
with bean-check failing like this:
```
#!

/tmp/foo.beancount:8: syntax error, unexpected KEY

/tmp/foo.beancount:9: syntax error, unexpected ACCOUNT

/tmp/foo.beancount:10: syntax error, unexpected KEY

/tmp/foo.beancount:4: Transaction does not balance: (-1230.27 USD)

2014-04-23 * "Flight to Berlin" #berlin-trip-2014
foo: "bar"
Expenses:Flights -1230.27 USD
```

According to some additional experiments I've made:
* no placement of full-line comments within a transaction works
* full-line comments to work between transactions though
* in-line comments work everywhere within transactions
* removing the leading whitespace (i.e., moving the ";" to column 0) does not fix the issue

I've only briefly looked at the yacc grammer, and it is not immediately clear why this does not work, as "posting_or_kv_list" entries are made of "key_value_line", which are properly terminated by "eol", which in turn can contain comments. I haven't debugged it any further though.