Remove the constants that assume a base unit in favor of explicit
x_per_y constants.
nanosecond calendar timestamps now use i128 for the type. This affects
fs.File.Stat, std.time.nanoTimestamp, and fs.File.updateTimes.
calendar timestamps are now signed, because the value can be less than
the epoch (the user can set their computer time to whatever they wish).
implement std.os.clock_gettime for Windows when clock id is
CLOCK_CALENDAR.
This rather large commit adds/fixes missing WASI functionality
in `libstd` needed to pass the `libstd` tests. As such, now by
default tests targeting `wasm32-wasi` target are enabled in
`test/tests.zig` module. However, they can be disabled by passing
the `-Dskip-wasi=true` flag when invoking the `zig build test`
command. When the flag is set to `false`, i.e., when WASI tests are
included, `wasmtime` with `--dir=.` is used as the default testing
command.
Since the majority of `libstd` tests were relying on `fs.cwd()`
call to get current working directory handle wrapped in `Dir`
struct, in order to make the tests WASI-friendly, `fs.cwd()`
call was replaced with `testing.getTestDir()` function which
resolved to either `fs.cwd()` for non-WASI targets, or tries to
fetch the preopen list from the WASI runtime and extract a
preopen for '.' path.
The summary of changes introduced by this commit:
* implement `Dir.makeDir` and `Dir.openDir` targeting WASI
* implement `Dir.deleteFile` and `Dir.deleteDir` targeting WASI
* fix `os.close` and map errors in `unlinkat`
* move WASI-specific `mkdirat` and `unlinkat` from `std.fs.wasi`
to `std.os` module
* implement `lseek_{SET, CUR, END}` targeting WASI
* implement `futimens` targeting WASI
* implement `ftruncate` targeting WASI
* implement `readv`, `writev`, `pread{v}`, `pwrite{v}` targeting WASI
* make sure ANSI escape codes are _not_ used in stderr or stdout
in WASI, as WASI always sanitizes stderr, and sanitizes stdout if
fd is a TTY
* fix specifying WASI rights when opening/creating files/dirs
* tweak `AtomicFile` to be WASI-compatible
* implement `os.renameatWasi` for WASI-compliant `os.renameat` function
* implement sleep() targeting WASI
* fix `process.getEnvMap` targeting WASI
