parser: add utf8 seeking functions

useful for peeking the previous utf8 char on a 0-copy utf8 buffer view

src/error.rs

@@ -1,6 +1,9 @@
+use crate::parser;
+
 #[derive(Eq, PartialEq, Debug)]
 pub enum Error {
     Nostr(enostr::Error),
+    Parse(parser::Error),
     Generic(String),
 }
 
@@ -10,6 +13,12 @@ impl From<String> for Error {
     }
 }
 
+impl From<parser::Error> for Error {
+    fn from(s: parser::Error) -> Self {
+        Error::Parse(s)
+    }
+}
+
 impl From<enostr::Error> for Error {
     fn from(err: enostr::Error) -> Self {
         Error::Nostr(err)