These changes make `HyREPL` use a single `HyASTCompiler` instance, instead of
creating one every time a valid source string is processed.
This change avoids the unnecessary re-initiation of the standard library
`import` and `require` steps that currently occur within the module tracked by a
`HyREPL` instance.
Also, one can now pass an existing compiler instance to `hy_repl` and
`hy_compiler`.
Closeshylang/hy#1698.