Fix nightly warnings

These will hit stable in about 6 weeks, it's nice to fix them early.

Note that this does not fix `clippy::upper_case_acronyms` since doing so
would be a breaking change. See
rust-lang/rust-clippy#6803 for more details.

Here are the warnings that were previously emitted:

```
warning: unnecessary trailing semicolon
    --> src/simple_api/mod.rs:1172:10
     |
1172 |         };
     |          ^ help: remove this semicolon
     |
     = note: `#[warn(redundant_semicolons)]` on by default
warning: panic message is not a string literal
   --> examples/threenp1.rs:127:30
    |
127 |             Err(x) => panic!(x),
    |                              ^
    |
    = note: `#[warn(non_fmt_panic)]` on by default
    = note: this is no longer accepted in Rust 2021
help: add a "{}" format string to Display the message
    |
127 |             Err(x) => panic!("{}", x),
    |                              ^^^^^
help: or use std::panic::panic_any instead
    |
127 |             Err(x) => std::panic::panic_any(x),
    |                       ^^^^^^^^^^^^^^^^^^^^^^
error: name `YDB` contains a capitalized acronym
   --> src/context_api/mod.rs:747:5
    |
747 |     YDB(YDBError),
    |     ^^^ help: consider making the acronym lowercase, except the initial letter: `Ydb`
    |
    = note: `-D clippy::upper-case-acronyms` implied by `-D clippy::all`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#upper_case_acronyms
```

Author: Joshua Nelson

examples/threenp1.rs

@@ -1,6 +1,6 @@
 /****************************************************************
 *                                                               *
-* Copyright (c) 2019-2020 YottaDB LLC and/or its subsidiaries.  *
+* Copyright (c) 2019-2021 YottaDB LLC and/or its subsidiaries.  *
 * All rights reserved.                                          *
 *                                                               *
 *       This source code contains the intellectual property     *
@@ -122,10 +122,7 @@ fn main() -> Result<(), Box<dyn Error>> {
         let start_time = SystemTime::now();
         // Wait for them to finish
         end_barrier.wait();
-        let time = match start_time.elapsed() {
-            Ok(elapsed) => elapsed.as_millis(),
-            Err(x) => panic!(x),
-        } as f64;
+        let time = start_time.elapsed().unwrap().as_millis() as f64;
 
         let updt = updates.get()?;
         let updt = String::from_utf8_lossy(&updt);

src/simple_api/mod.rs

@@ -1169,7 +1169,7 @@ impl Key {
             // SAFETY: This is only written to by `ydb_next_subscript` or `ydb_prev_subscript`, which only write variable names, not arbitrary data.
             // Since variable names are always ASCII, this is sound.
             this.subscripts.last_mut().unwrap_or(this.variable.as_mut_vec())
-        };
+        }
 
         let status = loop {
             // NOTE: this can't be hoisted out of the loop because the variable or subscripts could be resized on INVSTRLEN.