Janet 1.3.1

Mostly fixing bugs from 1.3.1, but also extends the length function and basic arithmetic and comparison functions to abstract types. This lets built in integer types behave more like normal numbers, and typed arrays behave more like normal arrays.

• Fix some linking issues when creating executables with native dependencies.
• jpm now runs each test script in a new interpreter.
• Fix an issue that prevent some valid programs from compiling.
• Add mean to core.
• Abstract types that implement the :+, :-, :*, :/, :>, :==, :<,
  :<=, and :>= methods will work with the corresponding built-in
  arithmetic functions. This means built-in integer types can now be used as
  normal number values in many contexts.
• Allow (length x) on typed arrays an other abstract types that implement
  the :length method.