Base64 encode the scope value

bkt permits setting a cache scope (provided via the `BKT_SCOPE`
environment variable or the `--scope` command line option) to prevent
collisions between unrelated command invocations. Previously, the scope
value was used verbatim in a directory path, causing errors if the scope
contained path-sensitive characters such as `/`.

This commit modifies bkt to base64 encode the scope value. Given a scope
value like `https://example.com/`, that means we now end up with a path
like:

    /tmp/bkt-0.7-cache/keys/aHR0cHM6Ly9leGFtcGxlLmNvbS8.C3823B193F1E0C78

Instead of the invalid:

    /tmp/bkt-0.7-cache/keys/https://www.example.com/.C3823B193F1E0C78

Fixes #50

Cargo.toml

@@ -28,6 +28,7 @@ bincode = "1.3.1"
 humantime = "2.1.0"
 rand = "0.8"
 serde = { version = "1.0", features = ["derive"] }
+base64 = "0.21.5"
 
 [dependencies.clap]
 version = "4.2"

src/lib.rs

@@ -28,6 +28,9 @@ use anyhow::{anyhow, Context, Error, Result};
 use serde::{Serialize, Deserialize};
 use serde::de::DeserializeOwned;
 
+use base64::{Engine as _, engine::general_purpose};
+
+
 #[cfg(feature="debug")]
 macro_rules! debug_msg {
     ($($arg:tt)*) => { eprintln!("bkt: {}", format!($($arg)*)) }
@@ -627,7 +630,7 @@ impl Cache {
 
     fn scoped(mut self, scope: String) -> Self {
         assert!(self.scope.is_none());
-        self.scope = Some(scope);
+        self.scope = Some(general_purpose::STANDARD_NO_PAD.encode(scope));
         self
     }