About 50% faster base64 implemention. (#12436)

tests/stdlib/tbase64.nim

@@ -0,0 +1,44 @@
+discard """
+  output: "OK"
+"""
+import base64
+
+proc main() =
+  doAssert encode("Hello World") == "SGVsbG8gV29ybGQ="
+  doAssert encode("leasure.") == "bGVhc3VyZS4="
+  doAssert encode("easure.") == "ZWFzdXJlLg=="
+  doAssert encode("asure.") == "YXN1cmUu"
+  doAssert encode("sure.") == "c3VyZS4="
+  doAssert encode([1,2,3]) == "AQID"
+  doAssert encode(['h','e','y']) == "aGV5"
+
+  doAssert encode("") == ""
+  doAssert decode("") == ""
+
+  const testInputExpandsTo76 = "+++++++++++++++++++++++++++++++++++++++++++++++++++++++++"
+  const testInputExpands = "++++++++++++++++++++++++++++++"
+  const longText = """Man is distinguished, not only by his reason, but by this
+    singular passion from other animals, which is a lust of the mind,
+    that by a perseverance of delight in the continued and indefatigable
+    generation of knowledge, exceeds the short vehemence of any carnal
+    pleasure."""
+  const tests = ["", "abc", "xyz", "man", "leasure.", "sure.", "easure.",
+                 "asure.", longText, testInputExpandsTo76, testInputExpands]
+
+  doAssert encodeMIME("foobarbaz", lineLen=4) == "Zm9v\r\nYmFy\r\nYmF6"
+  doAssert decode("Zm9v\r\nYmFy\r\nYmF6") == "foobarbaz"
+
+  for t in items(tests):
+    doAssert decode(encode(t)) == t
+    doAssert decode(encodeMIME(t, lineLen=40)) == t
+    doAssert decode(encodeMIME(t, lineLen=76)) == t
+
+  const invalid = "SGVsbG\x008gV29ybGQ="
+  try:
+    doAssert decode(invalid) == "will throw error"
+  except ValueError:
+    discard
+
+  echo "OK"
+
+main()