Fix#1020 (see more relevant discussion there)
This definitely makes the amount of generics compiler
has to generate by avoding generating the same functions
for `R`, &mut R`, `&mut &mut R` and so on.
old:
```
> ls -al target/release/deps/bitcoin-07a9dabf1f3e0266
-rwxrwxr-x 1 dpc dpc 9947832 Jun 2 22:42 target/release/deps/bitcoin-07a9dabf1f3e0266
> strip target/release/deps/bitcoin-07a9dabf1f3e0266
> ls -al target/release/deps/bitcoin-07a9dabf1f3e0266
-rwxrwxr-x 1 dpc dpc 4463024 Jun 2 22:46 target/release/deps/bitcoin-07a9dabf1f3e0266
```
new:
```
> ls -al target/release/deps/bitcoin-07a9dabf1f3e0266
-rwxrwxr-x 1 dpc dpc 9866800 Jun 2 22:44 target/release/deps/bitcoin-07a9dabf1f3e0266
> strip target/release/deps/bitcoin-07a9dabf1f3e0266
> ls -al target/release/deps/bitcoin-07a9dabf1f3e0266
-rwxrwxr-x 1 dpc dpc 4393392 Jun 2 22:45 target/release/deps/bitcoin-07a9dabf1f3e0266
```
In the unit-test binary itself, it saves ~100KB of data.
I did not expect much performance gains, but turn out I was wrong(*):
old:
```
test blockdata::block::benches::bench_block_deserialize ... bench: 1,072,710 ns/iter (+/- 21,871)
test blockdata::block::benches::bench_block_serialize ... bench: 191,223 ns/iter (+/- 5,833)
test blockdata::block::benches::bench_block_serialize_logic ... bench: 37,543 ns/iter (+/- 732)
test blockdata::block::benches::bench_stream_reader ... bench: 1,872,455 ns/iter (+/- 149,519)
test blockdata::transaction::benches::bench_transaction_deserialize ... bench: 136 ns/iter (+/- 3)
test blockdata::transaction::benches::bench_transaction_serialize ... bench: 51 ns/iter (+/- 8)
test blockdata::transaction::benches::bench_transaction_serialize_logic ... bench: 5 ns/iter (+/- 0)
test blockdata::transaction::benches::bench_transaction_size ... bench: 3 ns/iter (+/- 0)
```
new:
```
test blockdata::block::benches::bench_block_deserialize ... bench: 1,028,574 ns/iter (+/- 10,910)
test blockdata::block::benches::bench_block_serialize ... bench: 162,143 ns/iter (+/- 3,363)
test blockdata::block::benches::bench_block_serialize_logic ... bench: 30,725 ns/iter (+/- 695)
test blockdata::block::benches::bench_stream_reader ... bench: 1,437,071 ns/iter (+/- 53,694)
test blockdata::transaction::benches::bench_transaction_deserialize ... bench: 92 ns/iter (+/- 2)
test blockdata::transaction::benches::bench_transaction_serialize ... bench: 17 ns/iter (+/- 0)
test blockdata::transaction::benches::bench_transaction_serialize_logic ... bench: 5 ns/iter (+/- 0)
test blockdata::transaction::benches::bench_transaction_size ... bench: 4 ns/iter (+/- 0)
```
(*) - I'm benchmarking on a noisy laptop. Take this with a grain of salt. But I think
at least it doesn't make anything slower.
While doing all this manual labor that will probably generate conflicts,
I took a liberty of changing generic type names and variable names to
`r` and `R` (reader) and `w` and `W` for writer.
9c754ca4de