Split up arrow::array::builder module

[tustvold]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

The arrow::array::builder module is implemented as one immense builder.rs file, this has a number of drawbacks:

• Types have access to each others private fields, which can make invariants hard to reason about Implement UnionArray FieldData using Type Erasure #1842
• The file is incredibly unwieldy
• It is unclear what test coverage each of the builders have

Describe the solution you'd like

I would like to split out the various builders from builder.rs into their own submodules, e.g.

builder/mod.rs
builder/buffer_builder.rs
builder/boolean_builder.rs
...

The top-level builder/mod.rs can then re-export each of these with pub use to avoid introducing breaking changes.

Describe alternatives you've considered

We could not do this