Add main.rs to run all solutions and update README.md.

README.md

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+```
+Advent of code 2020
+Day 1 - Part 1: 1016964
+	generator: 22.481µs,
+	runner: 619ns
+
+Day 1 - Part 2: 182588480
+	generator: 16.507µs,
+	runner: 462ns
+
+Day 2 - Part 1: 586
+	generator: 150ns,
+	runner: 162.605µs
+
+Day 2 - Part 2: 352
+	generator: 147ns,
+	runner: 97.648µs
+
+Day 3 - Part 1: 265
+	generator: 63.238µs,
+	runner: 65.7µs
+
+Day 3 - Part 2: 3154761400
+	generator: 47.707µs,
+	runner: 544.894µs
+
+Day 4 - Part 2: 186
+	generator: 147ns,
+	runner: 133.651µs
+
+Day 5 - Part 1: SeatId(933)
+	generator: 114ns,
+	runner: 57.142µs
+
+Day 5 - Part 2: SeatId(711)
+	generator: 185ns,
+	runner: 56.998µs
+
+Day 6 - Part 1: 6633
+	generator: 111ns,
+	runner: 137.177µs
+
+Day 6 - Part 2: 3202
+	generator: 130ns,
+	runner: 116.545µs
+
+Day 7 - Part 1: 208
+	generator: 134ns,
+	runner: 716.627µs
+
+Day 7 - Part 2: 1664
+	generator: 135ns,
+	runner: 593.194µs
+
+```

src/main.rs

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+use aoc_runner_derive::aoc_main;
+
+aoc_main! { lib = aoc }