Unicode conversion rework

[traceon]

• Add ICU drop-in support (non-MSVC platforms only; system lib only)
• Factored out to dedicated headers: utils/amortized_istream_reader.h utils/resize_without_initialization.h utils/object_pool.h
• folly switched to a repo with my custom changes that enable resize-without-initialization to work on all target platforms
• Implemented: utils/string_pool.h utils/conversion_context.h utils/unicode_converter.h
• Separated utils/unicode_conv.h into: driver/utils/conversion.h (common code) driver/utils/conversion_std.h (default for MSVC) driver/utils/conversion_icu.h (default for all non-MSVC, not implemented for MSVC)
• Main conversion job is done in utils/unicode_converter.h, conversions are performed via pivot encoding, with some possible shortcuts, (hardcoded in ICU, UTF-16 placed in UChar arrays), main directions: application narrow-char <--(ICU pivot)--> driver internal narrow char application wide-char <--(ICU pivot)--> driver internal narrow char data source narrow-char <--(ICU pivot)--> driver internal narrow char
• This is a first step in transition to ICU-only implementation, so some code may seem redundant, or some further changes may seem like missing - most of them will take their places when the transition is done in subsequent iterations

[Enmk]

well, few questins:

driver/utils/conversion_std.h (MSVC only)

Why not conversion_msvc.h ?

conversions are performed via pivot encoding

What is pivot encoding here? Do I get it right, that you usually convert via some intermediate encoding? If yes, why?

[traceon]

Why not conversion_msvc.h ?

Because it can be still used with any other compiler, with one single macro switch.

What is pivot encoding here? Do I get it right, that you usually convert via some intermediate encoding? If yes, why?

ICU's converter-of-X only converts from X to its pivot, and from its pivot to X. ICU uses a hardcoded pivot, which is UTF-16 in UChar. Changing it to UTF-8 may speed up things in our case, but will require custom built ICU. This is planned for the next change. Everything inside the driver is represented in UTF-8.