Refactoring so that it can be implemented like a higher-order function without using `coswasmAPIImpl`

[da1suk8]

Since cosmwasmAPIGeneratorImpl was added in #41, and it is not good that cosmwasmAPIImpl and cosmwasmAPIGeneratorImpl coexist, I think it would be better to abolish cosmwasmAPIImpl and define it as a higher-order function and use that.　

wasmd/x/wasmplus/keeper/api.go

Lines 13 to 16 in d6ea9ec

	type cosmwasmAPIImpl struct {
	keeper *Keeper
	ctx *sdk.Context
	}