Propose methods to completely separate the functions of dynamic_link into wasmplus

[da1suk8]

At present #22, only partial separation has been made for dynamic link branch of wasmd, so consider how to do this.

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The following methods were used for implementation.

In #36, moved dynamic_link branch functionality to wasmplus.

It is implemented in the following way.

1. Define the interface of the function defined in x/wasmplus/keeper.go in x/wasm/keeper.go.
2. In x/wasm, functions and variables to be exposed to the outside world are defined as other variables in exported_*.go, in order to reduce differences from the original.
3. Access the functions defined in x/wasmplus/api.go via the interface in x/wasm/keeper.go.

[da1suk8]

Closed via #36