Convert string functions to UDF interface

This also switches to using the re2 library for regular expression
functions instead of boost.

Change-Id: I4c3ae72ff2f7cbd5b1a2be719275f1b2e25f8ab2
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/3412
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>
Tested-by: Skye Wanderman-Milne <skye@cloudera.com>
Expr refactoring

This patch changes the interface for evaluating expressions, in order
to allow for thread-safe expression evaluations and easier
codegen. Thread safety is achieved via the ExprContext class, a
light-weight container for expression tree evaluation state. Codegen
is easier because more expressions can be cross-compiled to IR.

See expr.h and expr-context.h for an overview of the API
changes. See sort-exec-exprs.cc for a simple example of the new
interface and hdfs-scanner.cc for a more complicated example.

This patch has not been completely code reviewed and may need further
cleanup/stylistic work, as well as additional perf work.

Change-Id: I3e3baf14ebffd2687533d0cc01a6fb8ac4def849
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/3459
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>
Tested-by: Skye Wanderman-Milne <skye@cloudera.com>