[Json] Proposal for strings/Unicode text

[Tim Bray <[e-mail]>]

Rationale:
- emphasize the important fact that Strings are *intended* for Unicode
characters
- document the important fact that the rules allow horrible Unicode
practices
- say “backslash” instead of “reverse solidus” :)

In section 1, introduction

Before:
   A string is a sequence of zero or more Unicode characters [UNICODE].
After:
   A string is intended to contain sequences of zero or more Unicode
characters [UNICODE 6.2]

Rewrite section 2.5 as follows:

Strings begin and end with quotation marks.  They are intended,to contain
sequences of Unicode characters; Note however that the ABNF in this section
allows the inclusion of 16-bit quantities in ways which can never be useful
for representing characters and is likely to cause breakage in software
designed to process Unicode text.

The ABNF allows the use of many Unicode code points that could be used in
future to represent Unicode characters, but have not yet been assigned.
Therefore, this specification should not need revision as the Unicode
character repertoire continues to grow.

16-bit quantities (normally Unicode characters from the Basic Multingual
Pane(U+0000 through U+FFFF) may be “escaped”, or represented as a
six-character sequence: a backslash (U+005C REVERSE SOLIDUS), followed  by
the lowercase letter u, followed by four hexadecimal digits that encode the
character's code point.  The hexadecimal letters A though F can be upper or
lower case.  So, for example, a string containing only a single backslash
may be represented as "\u005C".

Alternatively, there are two-character sequence escape representations of
some popular characters.  So, for example, a string containing only a
single backslash may be represented more compactly as "\\".

 To escape an extended character that is not in the Basic Multilingual
Plane, the character is represented as a twelve-character sequence,
encoding the UTF-16 surrogate pair.  So, for example, a string containing
only U+1D11E G CLEF may be represented as
   "\uD834\uDD1E".

=== insert ABNF here ====