The wast crate has a validate function (on Wats, Modules and Components), but:

• it's not getting called totally consistently (Wat parse calls it, but Module parse_without_module_keyword and Component parse_without_component_keyword don't call it, and those are used instead for some of the wast directives -- this led to some confusing debugging!)
• it doesn't do a ton (it checks for multiple start sections, which wasmparser is also capable of detecting)
• I think it would be safer to put all the eggs in wasmparser's basket to reduce the risk that there's an error condition that only gets caught when a module/component is expressed in the text format

This PR eliminates the validate function and defers detection of the error to the binary format.