fs: fully rooted, WASI, rewrite iterator

[tspader]

sp_sys_symlink now takes a kind parameter. Making a link to a directory and making a link to a file are fundamentally different operations at the OS level, and omitting this at the call site forced us to choose between sniffing or accepting broken behavior.

sp_sys_get_root_label returns the path of the Nth filesystem root on WASM, and nothing on native platforms.

Several functions which used to return a s64 length and signal errors via a negative sentinel now return sp_err_t

sp_sys_open_dir now accepts a flag, SP_SYS_OPEN_DIR_NO_FOLLOW, which closes the TOCTOU where a directory is swapped for a symlink mid-walk

We rewrote directory iteration to be layered:

• OS level, filling and parsing buffers with entries
• A basic iterator that hands you one at a time and asks the OS layer to refill when empty
• A high level iterator that handles the semantics of traversing directories

Further unfucking of errors

Basic WASM filesystem support

Fixed \ on POSIX platforms; it's now a legal character in a path

Stopped swallowing errors in the sp_fs_atomic