feat!: update get from (V, error) to (V, bool) (#20)

## Description

Currently, the signature for `Table.Get` is `func (K) (V, error)`. This is not very Go-idiomatic, which prefers to return a boolean instead of an error. For instance, a built-in Go map is used like so:

```go
if value, ok := users[id]; !ok {
  // ...
}
```

Updating our table to look like that is best practice. In that same vein, to support direct lookup (i.e. `v := users[id]`), this PR also adds `Table.Find`.

## Changes

- BREAKING CHANGE: Update contract of `Table.Get` to `func (K) (V, bool)`. Returns 'false' is the item cannot be found, and 'true' if it is found.
- Add `Table.Find`.
- Updated tests and documentation to match the change.

### Design Decisions

- Chose to make this decision because throwing an error implies that there is something 'wrong' with the table. There is nothing wrong with the table; it is just that the item does not exist.

## Checklist

- [x] Tests pass
- [x] Docs updated

Reviewed-on: #20
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

doc_example_test.go

@@ -14,19 +14,19 @@ func Example_basic() {
 		fmt.Println("Put error:", err)
 	}
 
-	if item, err := table.Get(1); err != nil {
-		fmt.Println("Error:", err)
+	if item, ok := table.Get(1); !ok {
+		fmt.Println("Not Found 1!")
 	} else {
 		fmt.Println("Found 1:", item)
 	}
 
-	if item, err := table.Get(0); err != nil {
-		fmt.Println("Error:", err)
+	if item, ok := table.Get(0); !ok {
+		fmt.Println("Not Found 0!")
 	} else {
 		fmt.Println("Found 0:", item)
 	}
 
 	// Output:
 	// Found 1: Hello, World!
-	// Error: key '0' not found
+	// Not Found 0!
 }