As if the Boston startup market needed additional momentum, it appears restaurant software startup Toast will dramatically bolster its valuation in its upcoming IPO. For a city perhaps best known internationally for its hard tech and biotech efforts, to see Toast not only rebound from its early-pandemic layoffs to a public debut, but to target a valuation closer to $20 billion than $10 billion, is a coup. The Exchange explores startups, markets and money. Read it every morning on Extra Crunch or get The Exchange newsletter every Saturday. In a…