Impled building account trie entries

Cleaned up and finished account parsing

Initial (buggy!) impl of trie insertion

Initial work on trie tests

Filled in trie `todo!()`s

First trie test now passes

Leaf splitting now works

- Just realized that I'm reading the keys backwards, so I'll need to
  deal with that.

.gitignore

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+Cargo.lock
+/**/target
+/target
+**/*.rs.bk
+*.iml
+.idea/
+.vscode

Cargo.toml

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+[package]
+name = "zero_eth_trie_utils"
+description = "Types and utility functions for building/working with partial Ethereum tries."
+version = "0.1.0"
+authors = ["Polygon Zero <[email protected]>"]
+edition = "2021"
+
+[dependencies]
+anyhow = "1.0.64"
+ethereum-types = "0.13.1"
+itertools = "0.10.3"
+log = "0.4.17"
+num-traits = "0.2.15"
+hex = "0.4.3"
+
+[dev-dependencies]
+pretty_env_logger = "0.4.0"

rustfmt.toml

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+unstable_features = true
+group_imports = "StdExternalCrate"
+wrap_comments = true

src/lib.rs

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+pub mod partial_trie;
+pub mod trie_builder;
+mod types;
+mod utils;

src/partial_trie.rs

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+use ethereum_types::U256;
+
+#[derive(Clone, Debug)]
+/// A partial trie, or a sub-trie thereof. This mimics the structure of an Ethereum trie, except
+/// with an additional `Hash` node type, representing a node whose data is not needed to process
+/// our transaction.
+pub enum PartialTrie {
+    /// An empty trie.
+    Empty,
+    /// The digest of trie whose data does not need to be stored.
+    Hash(U256),
+    /// A branch node, which consists of 16 children and an optional value.
+    Branch {
+        children: [Box<PartialTrie>; 16],
+        value: Option<U256>,
+    },
+    /// An extension node, which consists of a list of nibbles and a single child.
+    Extension {
+        nibbles: Nibbles,
+        child: Box<PartialTrie>,
+    },
+    /// A leaf node, which consists of a list of nibbles and a value.
+    Leaf { nibbles: Nibbles, value: Vec<u8> },
+}
+
+#[derive(Copy, Clone, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
+/// A sequence of nibbles.
+pub struct Nibbles {
+    /// The number of nibbles in this sequence.
+    pub count: usize,
+    /// A packed encoding of these nibbles. Only the first (least significant) `4 * count` bits are
+    /// used. The rest are unused and should be zero.
+    pub packed: U256,
+}