Tokenize emoji as if they were valid indentifiers

In the lexer, consider emojis to be valid identifiers and reject
them later to avoid knock down parse errors.

src/test/ui/parser/emoji-identifiers.rs

@@ -0,0 +1,16 @@
+struct ABig👩‍👩‍👧‍👧Family; //~ ERROR identifiers cannot contain emojis
+struct 👀; //~ ERROR identifiers cannot contain emojis
+impl 👀 {
+    fn full_of_✨() -> 👀 { //~ ERROR identifiers cannot contain emojis
+        👀
+    }
+}
+fn i_like_to_😅_a_lot() -> 👀 { //~ ERROR identifiers cannot contain emojis
+    👀::full_of✨() //~ ERROR no function or associated item named `full_of✨` found for struct `👀`
+    //~^ ERROR identifiers cannot contain emojis
+}
+fn main() {
+    let _ = i_like_to_😄_a_lot(); //~ ERROR cannot find function `i_like_to_😄_a_lot` in this scope
+    //~^ ERROR identifiers cannot contain emojis
+}
+

src/test/ui/parser/emoji-identifiers.stderr

@@ -0,0 +1,72 @@
+error[E0425]: cannot find function `i_like_to_😄_a_lot` in this scope
+  --> $DIR/emoji-identifiers.rs:13:13
+   |
+LL | fn i_like_to_😅_a_lot() -> 👀 {
+   | ----------------------------- similarly named function `i_like_to_😅_a_lot` defined here
+...
+LL |     let _ = i_like_to_😄_a_lot();
+   |             ^^^^^^^^^^^^^^^^^^ help: a function with a similar name exists: `i_like_to_😅_a_lot`
+
+error: identifiers cannot contain emojis: `i_like_to_😄_a_lot`
+  --> $DIR/emoji-identifiers.rs:13:13
+   |
+LL |     let _ = i_like_to_😄_a_lot();
+   |             ^^^^^^^^^^^^^^^^^^
+
+error: identifiers cannot contain emojis: `full_of_✨`
+  --> $DIR/emoji-identifiers.rs:4:8
+   |
+LL |     fn full_of_✨() -> 👀 {
+   |        ^^^^^^^^^^
+
+error: identifiers cannot contain emojis: `full_of✨`
+  --> $DIR/emoji-identifiers.rs:9:8
+   |
+LL |     👀::full_of✨()
+   |         ^^^^^^^^^
+
+error: identifiers cannot contain emojis: `👀`
+  --> $DIR/emoji-identifiers.rs:2:8
+   |
+LL | struct 👀;
+   |        ^^
+LL | impl 👀 {
+   |      ^^
+LL |     fn full_of_✨() -> 👀 {
+   |                        ^^
+LL |         👀
+   |         ^^
+...
+LL | fn i_like_to_😅_a_lot() -> 👀 {
+   |                            ^^
+LL |     👀::full_of✨()
+   |     ^^
+
+error: identifiers cannot contain emojis: `i_like_to_😅_a_lot`
+  --> $DIR/emoji-identifiers.rs:8:4
+   |
+LL | fn i_like_to_😅_a_lot() -> 👀 {
+   |    ^^^^^^^^^^^^^^^^^^
+
+error: identifiers cannot contain emojis: `ABig👩‍👩‍👧‍👧Family`
+  --> $DIR/emoji-identifiers.rs:1:8
+   |
+LL | struct ABig👩‍👩‍👧‍👧Family;
+   |        ^^^^^^^^^^^^^^^^^^
+
+error[E0599]: no function or associated item named `full_of✨` found for struct `👀` in the current scope
+  --> $DIR/emoji-identifiers.rs:9:8
+   |
+LL | struct 👀;
+   | ---------- function or associated item `full_of✨` not found for this
+...
+LL |     👀::full_of✨()
+   |         ^^^^^^^^^
+   |         |
+   |         function or associated item not found in `👀`
+   |         help: there is an associated function with a similar name: `full_of_✨`
+
+error: aborting due to 8 previous errors
+
+Some errors have detailed explanations: E0425, E0599.
+For more information about an error, try `rustc --explain E0425`.