Add ASDL description of the Fluent syntax.

ASDL is a language for describing ASTs.  It's a bit like structs and enums, but
more concise and language-agnostic.  It's different from (E)BNF in that it
doens't describe the grammar—but rather the data structure that can be used by
a compiler or an interpreter.

References:

    "The Zephyr Abstract Syntax Description Language"
    ftp://ftp.cs.princeton.edu/techreports/1997/554.pdf

    "Design of CPython’s Compiler"
    https://docs.python.org/devguide/compiler.html

    Using ASDL to describe ASTs in compilers
    http://eli.thegreenplace.net/2014/06/04/using-asdl-to-describe-asts-in-compilers

    "What is Zephyr ASDL?"
    http://www.oilshell.org/blog/2016/12/11.html

fluent.asdl

@@ -0,0 +1,31 @@
+module Fluent
+{
+    res = Resource(entry* body, comment? comment)
+
+    entry = Message(iden id, pat? value, mem* traits, comment? comment)
+          | Section(key key, entry* body, comment? comment)
+          | Comment(comment)
+          | Junk(string body)
+
+    pat = Pattern(elem* elements, bool quoted)
+    elem = Text(string)
+         | Placeable(expr* expressions)
+
+    expr = MessageReference(iden id)
+         | ExternalArgument(iden id)
+         | CallExpression(iden callee, expr* args)
+         | SelectExpression(expr exp, mem* vars)
+         | MemberExpression(expr obj, memkey key)
+         | KeyValueArgument(iden name, expr* val)
+         | Number(string value)
+         | String(string value)
+
+    mem = Member(memkey key, pat value, bool default)
+    memkey = Number(string value)
+           | Keyword(iden? ns, key name)
+
+    iden = (string name)
+    key = (string name)
+
+    comment = (string body)
+}