WASM SIMD integer to integer widening

• i16x8.widen_low_i8x16_s(a: v128) -> v128
• i16x8.widen_high_i8x16_s(a: v128) -> v128
• i16x8.widen_low_i8x16_u(a: v128) -> v128
• i16x8.widen_high_i8x16_u(a: v128) -> v128
• i32x4.widen_low_i16x8_s(a: v128) -> v128
• i32x4.widen_high_i16x8_s(a: v128) -> v128
• i32x4.widen_low_i16x8_u(a: v128) -> v128
• i32x4.widen_high_i16x8_u(a: v128) -> v128

The integer to integer widening instructions are essential to implement some heavily used OpenCV.js universal intrinsics, e.g. v_dotprod, v_reduce_sum and v_mul_expand.