Merge #12 #14

12: Add some S-Mode CSRs r=dvc94ch a=wangrunji0408

Add these S-Mode CSRs:
- `sstatus`
- `stvec`
- `sie`
- `sip`
- `scause`
- `stval`
- `sscratch`
- `sepc`
- `satp`

as well as:
- `time`
- `timeh`

and S-Mode instructions:
- `sfence.vma`

Most of the code have been tested in the [RustOS](https://github.com/wangrunji0408/RustOS) project.

14: Remove ecall and *ret instructions from riscv::asm r=dvc94ch a=Disasm

* *ret instructions should not be used directly in Rust code, they should be used in handlers, written in asm ([example](https://github.com/rust-embedded/riscv-rt/blob/273f0d4f70418a7602678632dcc78dbe24208fe2/src/lib.rs#L294-L340)

).
* ecall instruction should be wrapped into something like syscall(), which should be declared in another platform-specific crate.

Co-authored-by: WangRunji <wangrunji0408@163.com>
Co-authored-by: Vadim Kaushan <admin@disasm.info>