Add lexer support for `TOKEN_INTERPOLATION` to split string literals at interpolation points, introduce `MAX_INTERPOLATION_NESTING` limit of 8, and compile interpolated strings by emitting calls to `String.interpolate_()`. Optimize list and map literal construction with new `addCore_` primitives to reduce stack churn. Update `String`, `List`, and `Map` `toString` methods in core library to use interpolation syntax, and migrate all benchmark and test files from explicit concatenation to interpolated strings.
- Introduce wrenCallVarArgs as an explicit va_list variant, allowing variadic functions to forward their arguments directly without re-parsing.
- Fix a garbage collection issue in wrenCall where return values could be prematurely collected by ensuring proper root handling.
- Refactor the call API test to avoid re-entering the VM by moving test execution into an afterLoad callback instead of a foreign method.
Add a new 'a' format specifier to wrenCall that accepts an explicit byte array and length, enabling callers to pass strings containing null bytes without truncation. Also allow passing NULL for 'v' specifier arguments to produce a Wren NULL value. Rename setForeignCallbacks to setTestCallbacks for clarity. Extend the test suite with a new call.c test file that exercises all argument types including 'a' and 'v' with NULL, and add returnString/returnBytes foreign methods to verify wrenReturnString handles both null-terminated and explicit-length strings correctly.
