Add support for (either ...) in wast

[stevenfontanella]

• Adds support for (either ...) in assert_return in WAST
• Also allows customizing the error type in Result, which we use to keep track of the failing lane in assertions.

Example failure:

(module
  (func (export "f") (result i32)
    (i32.const 1)
  )
)

(assert_return (invoke "f") (either (i32.const 2) (i32.const 3)))

Expected one of (2 | 3) but got 1

Example failure with SIMD from relaxed_min_max.wast:

Expected one of (canonical f32 | canonical f32 | 0 | 0x00000000) at lane 0 but got i32x4 0x7fc00000 0x7fc00000 0x7fc00000 0x7fc00000

Part of #8261 and #8315. Fixes i16x8_relaxed_q15mulr_s.wast, i8x16_relaxed_swizzle.wast, relaxed_madd_nmadd.wast spec tests, and partially fixes relaxed_dot_product.wast, relaxed_laneselect.wast, relaxed_min_max.wast, and threads/thread.wast.

[tlively]

We should probably check that the value is not null here, too.

[tlively]

It's not clear to me why we check isAlternative here.