Remove Tree::{encode,decode}() in favor of Tree::{de}serialize()

[noib3]

Follow-up to #1.

TL;DR

Our custom binary encoding protocol is slightly more size efficient than serde + bincode, but both encoding and decoding are slower (benchmarks taken on a 2018 MacBook Pro).

I think it makes sense to remove the Encode and Decode traits in favor of the Tree::serialize() and Tree::deserialize() methods, which also have the added benefit of allowing users to use any serde data format they want.

Size comparison

Encode vs Serialize

Metadata = ()	Encode	Serialize w/ bincode
unit/small_tree/with_state	1,902 B	1,767 B
unit/small_tree/without_state	1,160 B	1,057 B
unit/medium_tree/with_state	26,173 B	31,129 B
unit/medium_tree/without_state	14,931 B	16,907 B
unit/large_tree/with_state	289,100 B	363,707 B
unit/large_tree/without_state	168,341 B	197,391 B

Metadata = "x".repeat(256)	Encode	Serialize w/ bincode
large_string/small_tree/with_state	27,960 B	27,926 B
large_string/small_tree/without_state	27,218 B	27,216 B
large_string/medium_tree/with_state	284,431 B	290,388 B
large_string/medium_tree/without_state	273,189 B	276,166 B
large_string/large_tree/with_state	2,869,358 B	2,953,966 B
large_string/large_tree/without_state	2,748,599 B	2,787,650 B

Speed comparison

Encode vs Serialize

Metadata = ()	Encode	Serialize w/ bincode
unit/small_tree/with_state	23.580 µs	6.5118 µs
unit/small_tree/without_state	14.385 µs	4.3177 µs
unit/medium_tree/with_state	228.70 µs	84.428 µs
unit/medium_tree/without_state	135.30 µs	41.110 µs
unit/large_tree/with_state	2.5599 ms	795.27 µs
unit/large_tree/without_state	1.4137 ms	363.39 µs

Metadata = "x".repeat(256)	Encode	Serialize w/ bincode
large_string/small_tree/with_state	27.216 µs	8.8061 µs
large_string/small_tree/without_state	21.076 µs	6.3444 µs
large_string/medium_tree/with_state	331.38 µs	127.20 µs
large_string/medium_tree/without_state	207.15 µs	95.048 µs
large_string/large_tree/with_state	3.5413 ms	1.8814 ms
large_string/large_tree/without_state	1.8826 ms	1.3542 ms

Decode vs Deserialize

Metadata = ()	Decode	Deserialize w/ bincode
unit/small_tree/with_state	15.823 µs	10.497 µs
unit/small_tree/without_state	15.353 µs	15.274 µs
unit/medium_tree/with_state	172.53 µs	137.49 µs
unit/medium_tree/without_state	154.04 µs	152.02 µs
unit/large_tree/with_state	1.6321 ms	1.1954 ms
unit/large_tree/without_state	1.7363 ms	1.7076 ms

Metadata = "x".repeat(256)	Decode	Deserialize w/ bincode
large_string/small_tree/with_state	49.856 µs	24.086 µs
large_string/small_tree/without_state	47.795 µs	26.973 µs
large_string/medium_tree/with_state	522.56 µs	260.29 µs
large_string/medium_tree/without_state	506.42 µs	293.34 µs
large_string/large_tree/with_state	5.2648 ms	2.6326 ms
large_string/large_tree/without_state	5.1263 ms	3.1588 ms