Improve wasm-c-api instantiation-time linking (#1902)

Add APIs to help prepare the imports for the wasm-c-api `wasm_instance_new`:
- wasm_importtype_is_linked
- wasm_runtime_is_import_func_linked
- wasm_runtime_is_import_global_linked
- wasm_extern_new_empty

For wasm-c-api, developer may use `wasm_module_imports` to get the import
types info, check whether an import func/global is linked with the above API,
and ignore the linking of an import func/global with `wasm_extern_new_empty`.

Sample `wasm-c-api-import` is added and document is updated.

samples/wasm-c-api-imports/wasm/CMakeLists.txt

@@ -0,0 +1,47 @@
+# Copyright (C) 2019 Intel Corporation.  All rights reserved.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+cmake_minimum_required (VERSION 3.14)
+project(wasm_modules)
+
+if(NOT SOCKET_WASI_CMAKE)
+  message(FATAL_ERROR "Require SOCKET_WASI_CMAKE")
+endif()
+
+option(WASM_TO_AOT "transfer wasm to aot" OFF)
+if(WASM_TO_AOT AND NOT WAMRC_PATH)
+  message(FATAL_ERROR "Require WAMRC_PATH when WASM_TO_AOT is ON")
+endif()
+
+#
+# c -> wasm
+include(${SOCKET_WASI_CMAKE})
+add_executable(send_recv ${CMAKE_CURRENT_LIST_DIR}/send_recv.c)
+set_target_properties(send_recv PROPERTIES SUFFIX .wasm)
+target_include_directories(send_recv PUBLIC ${CMAKE_CURRENT_LIST_DIR}/inc)
+target_link_libraries(send_recv socket_wasi_ext)
+target_link_options(send_recv PRIVATE
+  LINKER:--export=__heap_base
+  LINKER:--export=__data_end
+  LINKER:--shared-memory,--max-memory=196608
+  LINKER:--no-check-features
+  LINKER:--allow-undefined
+)
+
+if(WASM_TO_AOT)
+  # wasm -> aot
+  add_custom_target(send_recv_aot ALL
+    COMMAND pwd && ${WAMRC_PATH} --enable-multi-thread -o ./send_recv.aot ./send_recv.wasm
+    DEPENDS send_recv
+    WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
+  )
+endif()
+
+#
+# install
+if(WASM_TO_AOT)
+  install(FILES ${CMAKE_CURRENT_BINARY_DIR}/send_recv.aot DESTINATION . )
+else()
+  install(FILES ${CMAKE_CURRENT_BINARY_DIR}/send_recv.wasm DESTINATION . )
+endif()
+