Reorganize handling of task data

This abandons field-by-field compatibility with the TaskWarrior TDB2
format, which wasn't a sustainable strategy anyway.

Instead, tasks are represented as a TaskMap with custom key formats.  In
this commit, there are only a few allowed keys, with room to grow.

Replica returns convenience wrappers Task (read-only) and TaskMut
(read-write) with getters and setters to make modifying tasks easier.

Cargo.lock

@@ -131,12 +131,6 @@ dependencies = [
  "bitflags",
 ]
 
-[[package]]
-name = "either"
-version = "1.6.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e78d4f1cc4ae33bbfc157ed5d5a5ef3bc29227303d595861deb238fcec4e9457"
-
 [[package]]
 name = "failure"
 version = "0.1.6"
@@ -182,15 +176,6 @@ dependencies = [
  "wasi",
 ]
 
-[[package]]
-name = "itertools"
-version = "0.9.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "284f18f85651fe11e8a991b2adb42cb078325c996ed026d994719efcfca1d54b"
-dependencies = [
- "either",
-]
-
 [[package]]
 name = "itoa"
 version = "0.4.4"
@@ -622,7 +607,6 @@ dependencies = [
  "chrono",
  "clap",
  "failure",
- "itertools",
  "kv",
  "lmdb-rkv",
  "proptest",