Add timeout option to streambuf

[malachib]

This is decidedly non-standard C++ but important given our deviation in peek behavior

Since our peek is never blocking, things like cin >> ws are less useful. Therefore, make a feature flag which enables a timeoutable peek mode.

Upon doing this we might revisit the code deviation for sgetc behavior and make it blocking again (respecting the timeout) and perhaps implement a new non-standard speekc to feed our istream::peek. This retains the desired deviant behavior but very slightly closes the standards deviation gap itself.

Consider also adding an istream::inspect which I contend is a more invasive act than a peek, suggesting we retain non-standard non-blocking peek behavior and introduce inspect as the blocking version. Alternatively, come up with a verb even less invasive than peek (like maybe anticipate or nonblock_peek or something) and switch peek back to standard behavior

[malachib]

Code now treats peek as either:

1. Always-blocking (100% standardized behavior)
2. Timeout-blocking (non standard, but unlikely to break standard code run against it)

Timeout-blocking mode is enabled via FEATURE_IOS_TIMEOUT compiler define, and set it specifically to
the numeric value of milliseconds for the system level timeout. Though a compile-time timeout is somewhat restrictive, it should cover (with the addition of the nonstandard peek(timeout_ms) call) the use cases we're interested in.