Primitive types are too large, signed, and lack precision guarantees

Both `dart:ffi` and `dart:typed_data` expose differently sized storage for numeric types. However precision is only lost when transition from/to these storage areas; there is no way to maintain this imprecision within a Dart program. This results in bugs & unexpected behaviors. It also regresses runtime performance and memory usage because VM integers are assumed to be 64-bit, even where 32-bit or 16-bit operations would be more beneficial.

Having types that match the naming scheme of `typed_data` would help. To avoid accidental type punning the types could expose `toInt/toInt{prec}` and `toDouble/toDouble{prec}` methods, and `fromInt/fromInt{prec}` and `fromDouble/fromDouble{prec}` constructors. This would also mean these precise types do not extend `num`.

For floating point operations, the implementation could keep the 64-bit precision throughout all operations, but then store unboxed as the requested precision. This matches how Clang implements `__fp16` precision. Truncation between operations may be [prohibitively expensive](https://github.com/llvm/llvm-project/blob/e83e8e271169f06e78cd1b6c2bab5e2926f29ff1/compiler-rt/lib/builtins/fp_trunc_impl.inc#L41) (as is done with `_Float16`).

For integer types, the implementation could avoid truncation until observed. `int64` may end up using `BigInt` for JavaScript compilation so it should probably be truncated after each operation, but this is fast with `BigInt.asIntN/asUintN`. Unsigned integer types could be an extension type around the normal signed type.

This will also mean the discrepancy between the VM's 64-bit ints and JS's 52-bit ints would (mostly) disappear.

The issue names no file, test, or entry point. Start by reviewing the Dart language specification and the VM and JavaScript compilation concerns described here; done would require an agreed design for precise integer and floating-point types, conversions, storage, and operation semantics.

Written by the indexing model from the issue text.