Fix overflow panic in convert_interger_literal assist

This also seizes the opportunity to move integer literal parsing to
the syntax crate, were it logically belongs.

Note though that this is still done in an ad hoc manner -- we probably
should split kitchen sink ast::Literal into a separate APIs for
strings, ints, etc

crates/syntax/src/ast.rs

@@ -16,7 +16,7 @@ use crate::{
 };
 
 pub use self::{
-    expr_ext::{ArrayExprKind, BinOp, Effect, ElseBranch, LiteralKind, PrefixOp, RangeOp},
+    expr_ext::{ArrayExprKind, BinOp, Effect, ElseBranch, LiteralKind, PrefixOp, Radix, RangeOp},
     generated::{nodes::*, tokens::*},
     node_ext::{
         AttrKind, FieldKind, NameOrNameRef, PathSegmentKind, SelfParamKind, SlicePatComponents,