reorganize module

_archive/rhai_engine/rhaibook/rust/strings-interner.md

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+Strings Interner
+================
+
+{{#include ../links.md}}
+
+
+Because [strings] are immutable (i.e. the use the type [`ImmutableString`] instead of normal Rust `String`),
+each operation on a [string] actually creates a new [`ImmutableString`] instance.
+
+A _strings interner_ can substantially reduce memory usage by reusing the same [`ImmutableString`]
+instance for the same [string] content.
+
+An [`Engine`] contains a strings interner which is enabled by default
+(disabled when using a [raw `Engine`]).
+
+The maximum number of [strings] to be interned can be set via
+[`Engine::set_max_strings_interned`][options] (set to zero to disable the strings interner).