fix insert at beginning

[dnbln]

Previously trying to insert some code at the very beginning seems to have caused issues.

Using start=0, end=0 means "replace the first character with the given data", so to properly insert at the beginning the code should call .replace_range(0, -1, b"...") (not possible because -1 is not an usize). This commit fixes that by using exclusive ranges instead, so now .replace_range(0..0, b"...") inserts at the beginning.

[ehuss]

Thanks! I agree that all the saturating_sub stuff was a bit of a mess, and using exclusive seems to make more sense (and matches the JSON output, too). I'm a little uneasy since it can be difficult to reason about all the edge cases, but I don't see anything particularly concerning. Also, I ran it through the rustc testsuite and it seemed to be ok (though that is an unfortunately small set).