This commit reorganizes the stream-related logic by splitting the
`streams.rs` file into smaller, more manageable modules: `nodes.rs`,
`hooks.rs`, and `register.rs`.
The `nodes.rs` module now contains the core stream implementations like
`RootStream`, `PipeStream`, and the associated observer patterns.
The `hooks.rs` module encapsulates the compiler hook logic for `pipe`
and `pipe-series`, including type resolution and argument hinting. It
also includes helper functions for building pipe executors and
extracting stream information.
The `register.rs` module handles the registration of stream-related
built-in functions (`create-random-ohlc`, `create-ticker`, `pipe`,
`pipe-series`) within the environment.
This refactoring improves code organization, maintainability, and
testability by separating concerns into dedicated modules.
