Add voltage_within() check

[akhilles]

usage:

# child module
load("@stdlib/checks.zen", "voltage_within")
vbat = io("VBAT", Power, voltage_within("3.3–4.2V"))

# parent module
Child = Module("./child.zen")

VBAT = Power("VBAT", voltage="3.3–5V")
Child(name = "child", VBAT = VBAT)

^ example of what would cause an ERC check failure

[hexdae]

This is going to be awesome to add to the modules