xwasm: new package for dealing with WASM

[soypat]

add xwasm and use xwasm in bindiff

[github-actions]

memci size/alloc report wasm vs. main

Binary size: bindiff +94.96 KiB. Allocations unchanged.

Totals

target	base	head	Δ
bindiff	3.38 MiB	3.47 MiB	+94.96 KiB	+2.7%

Breakdown by package

bindiff — +94.96 KiB

go build -trimpath -buildvcs=false -o bindiff.elf ./cmd/bindiff

package	base	head	Δ
[unattributed]	2.19 MiB	2.24 MiB	+57.87 KiB	+2.6%
github.com/soypat/tinyboot/build/xwasm	—	16.88 KiB	+16.88 KiB	new
main	33.95 KiB	47.51 KiB	+13.56 KiB	+39.9%
go:func	163.07 KiB	166.95 KiB	+3.88 KiB	+2.4%
bytes	2.60 KiB	4.32 KiB	+1.73 KiB	+66.5%
internal/bytealg	1.28 KiB	1.93 KiB	+668 B	+51.1%
runtime	450.15 KiB	450.38 KiB	+242 B	+0.1%
type:	10.47 KiB	10.60 KiB	+133 B	+1.2%
go:itab	2.36 KiB	2.39 KiB	+32 B	+1.3%
go:main	184 B	192 B	+8 B	+4.3%

picobin — no change