Summary:
This commit brings following changes and fixes:

 * Implement parseExpr and compileParsedExpr;
 * Fix compileExpr and dynCompilerExpr, which returned `()` for empty expr;
 * Fix :def and :cmd, which didn't work if `IO` or `String` is not in scope;
 * Use GHCiMonad instead IO in :def and :cmd;
 * Clean PrelInfo: delete dead comment and duplicate entries, add assertion.

See new tests for more details.

Test Plan: ./validate

Reviewers: austin, dterei, simonmar

Reviewed By: simonmar

Subscribers: thomie, bgamari

Differential Revision: https://phabricator.haskell.org/D974

GHC Trac Issues: #10508