Implement AStringView as AByteBufferView

[Alex2772]

[Alex2772]

Issues

While implementing AStringView I have encountered the following design issues:

1. Should use char16_t instead of wchar_t because size of wchar_t differs on Windows and Linux, causing incompatibility.
2. Since AString is a wchar_t storage, AStringView is expected to be a wchar_t storage too, which forces to use widechar string literals (L"" instead of "").
3. Construction of AString from const char* always performs expensive utf8 parsing.

Possible solutions

1. Switch AString to basic char string and implement utf8 iterator, however text field and font rendering implementation may require a special 32-bit AString.
2. Use AStringView only for substrings and trims.
3. Implement string COW instead of AStringView

Existing solutions

1. Qt, C#, Java, Kotlin use UTF-16 in their strings, also Qt implements copy on write (COW).
2. GTK uses UTF-8.

[Alex2772]

Another possible solution: store a global std::unordered_map from const char* to const char16_t*.