stdlib std.os: Improve wasi-libc parity for WASI CWD emulation

Two major changes here:
  1. We store the CWD as a simple `[]const u8` and lookup Preopens for
     every absolute or CWD-referenced file operation, based on the
     Preopen with the longest match (i.e. most specific path)
  2. Preorders are normalized to POSIX absolute paths at init time.
     Behavior depends on the "cwd_root" parameter of `initPreopensWasi`:

	`cwd_root` is used for any Preopens that start with "."

	  For example:
            "./foo/bar" - inits to -> "{cwd_root}/foo/bar"
            "foo/bar"   - inits to -> "/foo/bar"
	    "/foo/bar"  - inits to -> "/foo/bar"

        `cwd_root` must be an absolute path.

	Using "/" as `cwd_root` gives behavior similar to wasi-libc.

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