Refactor public interface

- Rename `Settings` → `PanicPrinter`
- Make install, format and print functions methods of `PanicPrinter`
- Mostly backwards compatible with deprecation notices
  - Except for `Settings` that previously customized the `out` setting

examples/custom_message.rs

@@ -1,8 +1,5 @@
 fn main() {
     use color_backtrace::{default_output_stream, install_with_settings, Settings};
-    install_with_settings(
-        Settings::new().message("Custom message!"),
-        default_output_stream(),
-    );
+    install_with_settings(Settings::new().message("Custom message!"));
     assert_eq!(1, 2);
 }

examples/fmt_to_string.rs

@@ -1,9 +1,9 @@
-use color_backtrace::{format_backtrace, Settings};
+use color_backtrace::PanicPrinter;
 
 fn main() -> Result<(), std::io::Error> {
     let trace = backtrace::Backtrace::new();
-    let settings = Settings::default();
-    let str = format_backtrace(&trace, &settings)?;
+    let printer = PanicPrinter::default();
+    let str = printer.print_trace_to_string(&trace)?;
 
     if cfg!(windows) {
         println!(

examples/force_color.rs

@@ -20,8 +20,7 @@ fn fn1() {
 
 fn main() {
     use color_backtrace::{install_with_settings, Settings};
-    let out = termcolor::StandardStream::stderr(termcolor::ColorChoice::Always);
-    install_with_settings(Settings::new(), out);
+    install_with_settings(Settings::new());
 
     fn1();
 }