std/crypto/chacha20: add round-reduced versions & cleanup internals

See https://eprint.iacr.org/2019/1492.pdf for justification.

8 rounds ChaCha20 provides a 2.5x speedup, and is still believed
to be safe.

Round-reduced versions are actually deployed (ex: Android filesystem
encryption), and thanks to the magic of comptime, it doesn't take much
to support them.

This also makes the ChaCha20 code more consistent with the Salsa20 code,
removing internal functions that were not part of the public API any more.

No breaking changes; the public API remains backwards compatible.

lib/std/crypto.zig

@@ -24,8 +24,12 @@ pub const aead = struct {
     pub const Gimli = @import("crypto/gimli.zig").Aead;
 
     pub const chacha_poly = struct {
-        pub const ChaCha20Poly1305 = @import("crypto/chacha20.zig").Chacha20Poly1305;
-        pub const XChaCha20Poly1305 = @import("crypto/chacha20.zig").XChacha20Poly1305;
+        pub const ChaCha20Poly1305 = @import("crypto/chacha20.zig").ChaCha20Poly1305;
+        pub const ChaCha12Poly1305 = @import("crypto/chacha20.zig").ChaCha12Poly1305;
+        pub const ChaCha8Poly1305 = @import("crypto/chacha20.zig").ChaCha8Poly1305;
+        pub const XChaCha20Poly1305 = @import("crypto/chacha20.zig").XChaCha20Poly1305;
+        pub const XChaCha12Poly1305 = @import("crypto/chacha20.zig").XChaCha12Poly1305;
+        pub const XChaCha8Poly1305 = @import("crypto/chacha20.zig").XChaCha8Poly1305;
     };
 
     pub const isap = @import("crypto/isap.zig");
@@ -119,8 +123,14 @@ pub const sign = struct {
 pub const stream = struct {
     pub const chacha = struct {
         pub const ChaCha20IETF = @import("crypto/chacha20.zig").ChaCha20IETF;
+        pub const ChaCha12IETF = @import("crypto/chacha20.zig").ChaCha12IETF;
+        pub const ChaCha8IETF = @import("crypto/chacha20.zig").ChaCha8IETF;
         pub const ChaCha20With64BitNonce = @import("crypto/chacha20.zig").ChaCha20With64BitNonce;
+        pub const ChaCha12With64BitNonce = @import("crypto/chacha20.zig").ChaCha12With64BitNonce;
+        pub const ChaCha8With64BitNonce = @import("crypto/chacha20.zig").ChaCha8With64BitNonce;
         pub const XChaCha20IETF = @import("crypto/chacha20.zig").XChaCha20IETF;
+        pub const XChaCha12IETF = @import("crypto/chacha20.zig").XChaCha12IETF;
+        pub const XChaCha8IETF = @import("crypto/chacha20.zig").XChaCha8IETF;
     };
 
     pub const salsa = struct {

lib/std/crypto/benchmark.zig

@@ -202,6 +202,7 @@ pub fn benchmarkBatchSignatureVerification(comptime Signature: anytype, comptime
 const aeads = [_]Crypto{
     Crypto{ .ty = crypto.aead.chacha_poly.ChaCha20Poly1305, .name = "chacha20Poly1305" },
     Crypto{ .ty = crypto.aead.chacha_poly.XChaCha20Poly1305, .name = "xchacha20Poly1305" },
+    Crypto{ .ty = crypto.aead.chacha_poly.XChaCha8Poly1305, .name = "xchacha8Poly1305" },
     Crypto{ .ty = crypto.aead.salsa_poly.XSalsa20Poly1305, .name = "xsalsa20Poly1305" },
     Crypto{ .ty = crypto.aead.Gimli, .name = "gimli-aead" },
     Crypto{ .ty = crypto.aead.aegis.Aegis128L, .name = "aegis-128l" },