Refactor api code and doc

src/apis/emscripten/memory.rs

@@ -0,0 +1,25 @@
+use libc::{
+    c_void,
+    size_t,
+    memcpy,
+};
+
+use crate::webassembly::{Instance};
+
+/// emscripten: _emscripten_memcpy_big
+pub extern "C" fn _emscripten_memcpy_big(dest: u32, src: u32, len: u32, instance: &mut Instance) -> u32 {
+    let dest_addr = instance.memory_offset_addr(0, dest as usize) as *mut c_void;
+    let src_addr = instance.memory_offset_addr(0, src as usize) as *mut c_void;
+    unsafe { memcpy(dest_addr, src_addr, len as size_t); }
+    dest
+}
+
+/// emscripten: getTotalMemory
+pub extern "C" fn get_total_memory(instance: &mut Instance) -> u32 {
+    instance.memories[0].current_size()
+}
+
+/// emscripten: enlargeMemory
+pub extern "C" fn enlarge_memory() {
+    // stub
+}