Add raw string literal ambiguity document

src/grammar/raw-string-literal-ambiguity.md

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+Rust's lexical grammar is not context-free. Raw string literals are the source
+of the problem. Informally, a raw string literal is an `r`, followed by `N`
+hashes (where N can be zero), a quote, any characters, then a quote followed
+by `N` hashes. This grammar describes this as best possible:
+
+    R -> 'r' S
+    S -> '"' B '"'
+    S -> '#' S '#'
+    B -> . B
+    B -> ε
+
+Where `.` represents any character, and `ε` the empty string. Consider the
+string `r#""#"#`. This string is not a valid raw string literal, but can be
+accepted as one by the above grammar, using the derivation:
+
+    R : #""#"#
+    S : ""#"
+    S : "#
+    B : #
+    B : ε
+
+(Where `T : U` means the rule `T` is applied, and `U` is the remainder of the
+string.) The difficulty arises from the fact that it is fundamentally
+context-sensitive. In particular, the context needed is the number of hashes.
+I know of no way to resolve this, but also have not come up with a proof that
+it is not context sensitive. Such a proof would probably use the pumping lemma
+for context-free languages, but I (cmr) could not come up with a proof after
+spending a few hours on it, and decided my time best spent elsewhere. Pull
+request welcome!