refactor(3p/nix): Remove custom base64 implementation

Replace the custom, rather questionable base64 implementation with
absl::Base64{Une,E}scape. To make sure that the custom implementation
was doing the same thing I've also added a test covering
nix::Hash::to_string, which was one function that used it - the test
passed prior to the replacement, and continued to pass afterwards.

The previous base64Decode function threw an exception on failure - to
avoid going too far down the rabbit hole I've replicated that
functionality at all call sites, but this should be replaced with more
sensible error handling such as StatusOr eventually.

Also, before this change:

    ❯ nix eval -f . users.tazjin.emacs.outPath
    "/nix/store/g6ri2q8nra96ix20bcsc734r1yyaylb1-tazjins-emacs"

And after:

    ❯ ./result/bin/nix eval -f . users.tazjin.emacs.outPath
    "/nix/store/g6ri2q8nra96ix20bcsc734r1yyaylb1-tazjins-emacs"

Change-Id: Id292ffbb82fe808f3f1b34670afbe7b8c13ad615
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1385
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

third_party/nix/src/tests/CMakeLists.txt

@@ -11,6 +11,14 @@ target_link_libraries(attr-set
 
 gtest_discover_tests(attr-set)
 
+add_executable(hash_test hash_test.cc)
+target_link_libraries(hash_test
+  nixutil
+  GTest::gtest_main
+)
+
+gtest_discover_tests(hash_test)
+
 add_executable(value-to-json value-to-json.cc)
 target_link_libraries(value-to-json
   nixexpr

third_party/nix/src/tests/hash_test.cc

@@ -0,0 +1,24 @@
+#include "libutil/hash.hh"
+
+#include <gtest/gtest.h>
+
+class HashTest : public ::testing::Test {};
+
+namespace nix {
+
+TEST(HASH_TEST, SHA256) {
+  auto hash = hashString(HashType::htSHA256, "foo");
+  ASSERT_EQ(hash.base64Len(), 44);
+  ASSERT_EQ(hash.base32Len(), 52);
+  ASSERT_EQ(hash.base16Len(), 64);
+
+  ASSERT_EQ(hash.to_string(Base16),
+            "sha256:"
+            "2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae");
+  ASSERT_EQ(hash.to_string(Base32),
+            "sha256:1bp7cri8hplaz6hbz0v4f0nl44rl84q1sg25kgwqzipzd1mv89ic");
+  ASSERT_EQ(hash.to_string(Base64),
+            "sha256:LCa0a2j/xo/5m0U8HTBBNBNCLXBkg7+g+YpeiGJm564=");
+}
+
+}  // namespace nix