ra_syntax: refactored the lexer design as per @matklad and @kiljacken PR review

crates/ra_syntax/src/lib.rs

@@ -41,7 +41,9 @@ use crate::syntax_node::GreenNode;
 pub use crate::{
     algo::InsertPosition,
     ast::{AstNode, AstToken},
-    parsing::{first_token, single_token, tokenize, tokenize_append, Token, TokenizeError},
+    parsing::{
+        lex_single_syntax_kind, lex_single_valid_syntax_kind, tokenize, Token, TokenizeError,
+    },
     ptr::{AstPtr, SyntaxNodePtr},
     syntax_error::{Location, SyntaxError, SyntaxErrorKind},
     syntax_node::{