nix-hash: support base-64 and SRI format

Add the --base64 and --sri flags for the Base64 and SRI format output.

Add the --base16 flag to explicitly specify the hexadecimal format.

Add the --to-base64 and --to-sri flag to convert a hash to the above
mentioned format.

doc/manual/src/command-ref/nix-hash.md

@@ -6,9 +6,7 @@
 
 `nix-hash` [`--flat`] [`--base32`] [`--truncate`] [`--type` *hashAlgo*] *path…*
 
-`nix-hash` `--to-base16` *hash…*
-
-`nix-hash` `--to-base32` *hash…*
+`nix-hash` [`--to-base16`|`--to-base32`|`--to-base64`|`--to-sri`] [`--type` *hashAlgo*] *hash…*
 
 # Description
 
@@ -35,11 +33,23 @@ md5sum`.
     The result is identical to that produced by the GNU commands
     `md5sum` and `sha1sum`.
 
+  - `--base16`\
+    Print the hash in a hexadecimal representation (default).
+
   - `--base32`\
     Print the hash in a base-32 representation rather than hexadecimal.
     This base-32 representation is more compact and can be used in Nix
     expressions (such as in calls to `fetchurl`).
 
+  - `--base64`\
+    Similar to --base32, but print the hash in a base-64 representation,
+    which is more compact than the base-32 one.
+
+  - `--sri`\
+    Print the hash in SRI format with base-64 encoding.
+    The type of hash algorithm will be prepended to the hash string,
+    followed by a hyphen (-) and the base-64 hash body.
+
   - `--truncate`\
     Truncate hashes longer than 160 bits (such as SHA-256) to 160 bits.
 
@@ -55,6 +65,14 @@ md5sum`.
     Don’t hash anything, but convert the hexadecimal hash representation
     *hash* to base-32.
 
+  - `--to-base64`\
+    Don’t hash anything, but convert the hexadecimal hash representation
+    *hash* to base-64.
+
+  - `--to-sri`\
+    Don’t hash anything, but convert the hexadecimal hash representation
+    *hash* to SRI.
+
 # Examples
 
 Computing the same hash as `nix-prefetch-url`:
@@ -81,9 +99,18 @@ $ nix-store --dump test/ | md5sum (for comparison)
 $ nix-hash --type sha1 test/
 e4fd8ba5f7bbeaea5ace89fe10255536cd60dab6
 
+$ nix-hash --type sha1 --base16 test/
+e4fd8ba5f7bbeaea5ace89fe10255536cd60dab6
+
 $ nix-hash --type sha1 --base32 test/
 nvd61k9nalji1zl9rrdfmsmvyyjqpzg4
 
+$ nix-hash --type sha1 --base64 test/
+5P2Lpfe76upazon+ECVVNs1g2rY=
+
+$ nix-hash --type sha1 --sri test/
+sha1-5P2Lpfe76upazon+ECVVNs1g2rY=
+
 $ nix-hash --type sha256 --flat test/
 error: reading file `test/': Is a directory
 
@@ -91,7 +118,7 @@ $ nix-hash --type sha256 --flat test/world
 5891b5b522d5df086d0ff0b110fbd9d21bb4fc7163af34d08286a2e846f6be03
 ```
 
-Converting between hexadecimal and base-32:
+Converting between hexadecimal, base-32, base-64, and SRI:
 
 ```console
 $ nix-hash --type sha1 --to-base32 e4fd8ba5f7bbeaea5ace89fe10255536cd60dab6
@@ -99,4 +126,13 @@ nvd61k9nalji1zl9rrdfmsmvyyjqpzg4
 
 $ nix-hash --type sha1 --to-base16 nvd61k9nalji1zl9rrdfmsmvyyjqpzg4
 e4fd8ba5f7bbeaea5ace89fe10255536cd60dab6
+
+$ nix-hash --type sha1 --to-base64 e4fd8ba5f7bbeaea5ace89fe10255536cd60dab6
+5P2Lpfe76upazon+ECVVNs1g2rY=
+
+$ nix-hash --type sha1 --to-sri nvd61k9nalji1zl9rrdfmsmvyyjqpzg4
+sha1-5P2Lpfe76upazon+ECVVNs1g2rY=
+
+$ nix-hash --to-base16 sha1-5P2Lpfe76upazon+ECVVNs1g2rY=
+e4fd8ba5f7bbeaea5ace89fe10255536cd60dab6
 ```