Let String pass #[track_caller] to its Vec calls

[kornelski]

I've added #[track_caller] to String methods that delegate to Vec methods that already have #[track_caller].

I've also added #[track_caller] to methods that have assert! or panic! due to invalid inputs.

[rustbot]

r? @tgross35

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[tgross35]

Since this sometimes increases compile times

@bors2 try
@rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[rust-bors]

⌛ Trying commit c109b28 with merge 075cf9a…

To cancel the try build, run the command @bors2 try cancel.

[rust-bors]

☀️ Try build successful (CI)
Build commit: 075cf9a (075cf9a3e6ff4d06acf01becd0b8e06b926329e1, parent: 2fcf1776b9ccef89993dfe40e9f5c4908e2d2d48)

[rust-timer]

Queued 075cf9a with parent 2fcf177, future comparison URL.
There are currently 6 preceding artifacts in the queue.
It will probably take at least ~8.7 hours until the benchmark run finishes.

[tgross35]

Mind posting a difference in output with and without this change for reference?