Add managed string and array reads

[ero-qt]

Part of #149.

Adds read_string and read_array to both backends' Module. Without them, reading a System.String or a managed array is a fixed-size read at a memorized header offset, unchecked against the count and wrong at the other pointer width.

read_string reads the character count, then exactly that many UTF-16 characters. A count past the buffer, a negative count, and a null reference fail rather than truncate.

read_array does the same with the length, judged at pointer width before narrowing so a huge value cannot slip through as its low half. The element type is the caller's claim, like Process::read. Managed char is u16, and Rust char never matches.

Both take the address the reference is stored at and dereference at the module's own width, so 32-bit games need no special handling.

ArrayVec appears in a public signature for the first time. The crate already re-exports arrayvec.

cargo test --all-features runs the suite. The new tests cover strings and arrays on both runtimes, counts past the buffer, lengths at full width, and 32-bit targets.

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Depends on #146.