Port trace log from wasmtime-wasi-c

[kubkon]

This PR builds directly on #27 (which in turn builds on #26). It ports the trace logs from wasmtime-wasi-c/src/syscalls.rs for easier debugging.

EDIT: Example usage:

Assuming you're using wasi-common together with wasmtime, the logs can be enabled directly from the terminal by running:

$ RUST_LOG=wasi_common=trace wasmtime -d --wasi-common ...

Producing an elegant output of the form:

 TRACE wasi_common::hostcalls::fs > fd_prestat_get(fd=3, prestat_ptr=0xffff0)
 TRACE wasi_common::hostcalls     >     -> errno=__WASI_ESUCCESS
 TRACE wasi_common::hostcalls::fs > fd_prestat_dir_name(fd=3, path_ptr=0x110088, path_len=1)
 TRACE wasi_common::hostcalls::fs >      | (path_ptr,path_len)="."
 TRACE wasi_common::hostcalls     >     -> errno=__WASI_ESUCCESS
 TRACE wasi_common::hostcalls::fs > fd_fdstat_get(fd=3, fdstat_ptr=0xfffd8)
 TRACE wasi_common::hostcalls::fs >      | *buf=__wasi_fdstat_t { fs_filetype: 3, fs_flags: 0, fs_rights_base: 264240792, fs_rights_inheriting: 268435455 }
 TRACE wasi_common::hostcalls     >     -> errno=__WASI_ESUCCESS
 TRACE wasi_common::hostcalls::fs > fd_prestat_get(fd=4, prestat_ptr=0xffff0)
 TRACE wasi_common::hostcalls     >     -> errno=__WASI_EBADF
 TRACE wasi_common::hostcalls::misc > environ_sizes_get(environ_count_ptr=0xffff0, environ_size_ptr=0xffffc)
 TRACE wasi_common::hostcalls::misc >      | *environ_count_ptr=0
 TRACE wasi_common::hostcalls::misc >      | *environ_size_ptr=0
 TRACE wasi_common::hostcalls       >     -> errno=__WASI_ESUCCESS
 TRACE wasi_common::hostcalls::misc > args_sizes_get(argc_ptr=0xffffc, argv_buf_size_ptr=0xffff0)
 TRACE wasi_common::hostcalls::misc >      | *argc_ptr=2
 TRACE wasi_common::hostcalls::misc >      | *argv_buf_size_ptr=18
 TRACE wasi_common::hostcalls       >     -> errno=__WASI_ESUCCESS
 TRACE wasi_common::hostcalls::misc > args_get(argv_ptr=0x110088, argv_buf=0x1100a8)
 TRACE wasi_common::hostcalls       >     -> errno=__WASI_ESUCCESS
 TRACE wasi_common::hostcalls::misc > args_sizes_get(argc_ptr=0xffe58, argv_buf_size_ptr=0xffe5c)
 TRACE wasi_common::hostcalls::misc >      | *argc_ptr=2
 TRACE wasi_common::hostcalls::misc >      | *argv_buf_size_ptr=18
 TRACE wasi_common::hostcalls       >     -> errno=__WASI_ESUCCESS
 TRACE wasi_common::hostcalls::misc > args_get(argv_ptr=0x110118, argv_buf=0x110128)
 TRACE wasi_common::hostcalls       >     -> errno=__WASI_ESUCCESS
...

[sunfishcode]

The trace parts here look great! This is just blocked on #27.

[kubkon]

The trace parts here look great! This is just blocked on #27.

Yeah, that's my fault. Looking back I probably shouldn't have made this PR dependent on #27, but then I thought it was a good idea to build on it and here we are.

[sunfishcode]

#27 is now merged; would you mind rebasing the merge conflicts here?

[kubkon]

Since #27 is now merged, I guess we can merge this one as well.