README: update bundle sizes, clarify what bundle sizes mean

[slimsag]

• Add TinyGo bundle sizes which shows the clear benefit of TinyGo.
• Updates numbers to Go 1.15 compiler, which produces much smaller bundles.
• Add brotli compression numbers.
• Clarify that these numbers are baseline and do not scale linearly with dependencies.
• Change "Small bundle sizes" to "Near-zero dependencies" to clarify we (on the Vecty side) are doing everything we can and majority of the size comes from the Go compilers. Also, this eliminates anyone's concern about "several MB is small?!" (which it's not.)