* 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.
