refactor!: shorter constructors, bucket → subtable (#22)

## Description

Currently, the name of `bucket` is a bit confusing, because it is considered a 'table' in literature (as well as the whole hash table). A `bucket` is better described as a 'subtable', which is used by the total hash table to perform cuckoo hashing.

In addition, the constructors `NewTable`, `NewTableBy`, and `NewCustomTable` were given shorter names, because the package name `cuckoo` already implies that `New*` would create a hash table with cuckoo hashing. This package has one use-case, and so it unambiguous what constructors produce.

## Changes

- `NewTable` -> `New`
- `NewTableBy` -> `NewBy`
- `NewCustomTable` -> `NewCustom`
- `bucket` -> `subtable`

### Design Decisions

- I would have renamed `Table` and `subtable` to map equivalents, but 'submap' implies that a certain subsection of the map is contained within it, which isn't quite right.
- I chose not to go with `Map` and `table`, because of the split naming convention.

## Checklist

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

Reviewed-on: #22
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

doc.go

@@ -1,8 +1,8 @@
 // Package cuckoo provides a hash table that uses cuckoo hashing to achieve
 // a worst-case O(1) lookup time.
 //
-// While a [NewTable] only supports comparable keys by default, you can create
-// a table with any key type using [NewCustomTable]. Custom [Hash] functions and
+// While a [New] only supports comparable keys by default, you can create
+// a table with any key type using [NewCustom]. Custom [Hash] functions and
 // key comparison are also supported.
 //
 // See more: https://en.wikipedia.org/wiki/Cuckoo_hashing