base64: some optimizations

* Add a test for the base64 encoding/decoding of long strings (i.e. mainly memory allocation).

* Make vlib/encoding/base64/base64_memory_test.v resemble more test.v from https://github.com/kostya/benchmarks .

* base64: some optimizations, also add base64.encode_in_buffer and base64.decode_in_buffer .

* Fix tests passing static strings.

* Reduce time needed for base64_memory_test.v .

* Optimize encoding.base64.Index access too (it is static), which speeds up decoding.

vlib/encoding/base64/base64_memory_test.v

@@ -0,0 +1,32 @@
+import encoding.base64
+
+fn test_long_encoding(){
+	repeats := 1000
+	input_size := 3000
+	
+	s_original := 'a'.repeat(input_size)
+	s_encoded := base64.encode(s_original)
+	s_decoded := base64.decode(s_encoded)
+	
+	assert s_encoded.len > s_original.len
+	assert s_original == s_decoded
+
+	mut s := 0
+	
+	ebuffer := malloc( s_encoded.len )
+	for i := 0; i < repeats; i++ {
+		resultsize := base64.encode_in_buffer(s_original, mut ebuffer)
+		s += resultsize
+		assert resultsize == s_encoded.len
+	}
+
+	dbuffer := malloc( s_decoded.len )
+	for i := 0; i < repeats; i++ {
+		resultsize := base64.decode_in_buffer(s_encoded, mut dbuffer)
+		s += resultsize
+		assert resultsize == s_decoded.len
+	}
+
+	println( 'Final s: $s' )
+	//	assert s == 39147008
+}