Use try blocks for error handling in lib.rs

[Copilot]

Refactor error handling to use Rust's nightly try_blocks feature as requested.

Changes

• Add #![feature(try_blocks)] crate attribute
• Add rust-toolchain.toml to specify nightly channel
• Refactor next_states, memory_state_from_sm2, and memory_state methods to use try block pattern

Example

Before:

self.0.lock().unwrap().next_states(...).unwrap()

After:

(try {
    self.0.lock().unwrap().next_states(...)?
} as Result<_, fsrs::FSRSError>).unwrap()

Original prompt

---

This section details on the original issue you should resolve

<issue_title>try try</issue_title>
<issue_description>

fsrs-rs-python/src/lib.rs

Lines 24 to 34 in 798fcba

	NextStates(
	self.0
	.lock()
	.unwrap()
	.next_states(
	current_memory_state.map(|x| x.0),
	desired_retention,
	days_elapsed,
	)
	.unwrap(),
	)

use try block to rewrite them as

try {
   x?
   y?
}.unwrap()

Can use nightly.
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes try try #48

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