ViperJS 0.2.0

The first released version. 0.1.0 was never published; this is what the crate has grown into
since, and the headline is that the engine runs JavaScript rather than merely reading it.

Added

• Lexer. Every token form ECMA-262 §12 defines: identifiers over the real Unicode
  ID_Start/ID_Continue sets, all numeric literals including Annex B's legacy forms and
  BigInt, strings and templates as UTF-16 code units, regular-expression literals under a goal
  symbol supplied by the parser, and all four line terminators. Every token knows its exact span,
  and the token spans plus the trivia between them reconstruct the source byte for byte.
• Parser. Statements and declarations, the full expression grammar with correct precedence
  and associativity, automatic semicolon insertion, functions, classes with fields, static blocks
  and private names, generators, async/await, arrow functions, destructuring and the three
  cover grammars, template literals, optional chaining, and modules — each with the early errors
  the spec demands. Errors are values carrying spans, and read like a good compiler's.
• Values, objects and a garbage collector. The object model with prototypes, property
  descriptors, getters and setters and the ordinary internal methods; a mark-sweep collector with
  slot reuse behind generation-tagged handles, scheduled by the interpreter itself.
• A bytecode compiler and interpreter. Block scoping with the temporal dead zone, closures,
  this, try/catch/finally, and the abstract operations spelled out as such — ToNumber,
  ToString, ToPrimitive, ToPropertyKey.
• The ES5 library, complete. Object, Function, Array, String, Number, Boolean,
  Math, JSON, Date, the Error hierarchy, and our own backtracking RegExp engine —
  no dependency — including named groups, lookbehind, the u and v flags and Annex B's §B.1.2
  grammar.
• ES2015 and beyond. Classes, iterators and generators, Symbol, Map/Set/WeakMap/
  WeakSet, Promise with §9.5's job queue, Proxy and Reflect, ArrayBuffer, DataView
  and the TypedArrays including the resizable-buffer semantics, async functions and await,
  async generators, BigInt, and modules — live bindings, namespace objects, export * with its
  ambiguity rule, dynamic import() and top-level await.
• eval in both modes, with, and Annex B. Direct eval resolves into the scopes its
  caller is running in; §B.3's block-level function declarations and §B.2's library additions are
  implemented for sloppy code.
• An embedding surface (api.rs): Engine owns the heap and the machine together, host
  functions can be bound from Rust, and a stale handle is refused rather than read as undefined.
  examples/embed.rs is the tour.
• An interruptible run. Engine::set_time_budget stops a script that will not stop, and
  because a budget a script can catch is not a budget, exceeding it is not a throw.
• A viper command line — run a file, a string with -e, or standard input.
• A test262 harness (conformance/) with an expectations file that may only shrink.

Conformance

About 84% of test262 at this release — 78,222 of 93,161 runs. Treat that number as
perishable and re-measure rather than quoting it. The largest remaining gaps are proposals rather
than the standard: Temporal, explicit resource management, decorators, and Atomics tests
needing multiple agents.

Guarantees held throughout

• Zero runtime dependencies; the dependency table is empty and checked in CI.
• #![forbid(unsafe_code)], crate-wide.
• No input panics. Nesting is bounded by an explicit count rather than by hitting the OS stack
  guard, and a full-depth parse is asserted to survive one mebibyte of stack.