Removed non-streaming h2 frame decoder.

Remove whole-frame h2 decoder (#179)

We use the streaming decoder now. Most changes are rewriting tests that assumed the encoder/decoder were mirrors of each other.

**REMOVE**
- `aws_h2_frame_decoder_***()` functions
- `fuzz_h2_frames.c` which just tested the old decoder.

**ALTER**
- Rewrite encoder tests (delete `test_h2_frames.c`, add `test_h2_encoder.c`). Old tests relied on whole-frame decoder.
- Alter `test_h2_headers.c` to use new streaming decoder.
- Add `aws_h2_encode_frame()` function that accepts all frames types, instead of needing to call the appropriate aws_h2_frame_XYZ_encode() function per frame type.

**FIX**
- Fix encoder bug with padding in DATA frame.
- Fix decoder bug where header-value-string overwrote header-name-string.