:bug: Libc is not thread safe

#### Expected Behavior
Libc should be thread safe.

#### Observed Behavior
Libc is not actually thread safe, because ZestCode does not implement the necessary locking routines: https://github.com/bminor/newlib/blob/master/newlib/libc/misc/lock.c.

#### Steps to Reproduce
n/a

#### Environment
n/a

#### Additional Information
Before this can be fixed, the root cause of #16 needs to be determined.