Add more arrows such as `\mapsto`, `\leadsto`

The following arrows are added:
`\Leftarrow`, `\longleftarrow`, `\leftrightarrow`, `\longleftrightarrow`,
`\Rightarrow`, `\longrightarrow`, `\leadsto`, `\mapsto`, `\longmapsto`
I tried this out, I think this needs to be the `"ams"` in order to work.