Move files from src/ to src/nvim/.
- src/nvim/ becomes the new root dir for nvim executable sources.
- src/libnvim/ is planned to become root dir of the neovim library.

reverting broad cosmetic/style change because:
- increases merge-conflicts
- increases overhead of merging upstream Vim patches
- reasons for change are ambiguous, so default to no change

Use `xcalloc` instead.
Inline `alloc_tv` and `alloc_string_tv` in eval.c

Today's compilers generate shift instructions to perform division and
multiplications by powers of 2 [1]. `(x >> 1)` looks straightforward enough, but
if x is signed the code will fail when x < 0. The compiler knows better: use
`x / 2`.

That's why we have code like this:

    (long)((long_u)Rows >> 1))

instead of the cleaner version that generates the same or better machine code:

    Rows / 2

[1] http://goo.gl/J4WpG7

 - alloc_tabpage()
 - win_alloc_lines()
 - win_alloc_aucmd_win()
 - new_frame()
 - win_alloc()

TODO: don't handle OOM after calls to dict_alloc()

Fix uses of plain "%ld" within EMSGN():
- Replace "%ld" with "%" PRId64.
- No argument cast needed. EMSGN() will take care of that.

Fix uses of plain "%ld" within vim_snprintf():
- Replace "%ld" with "%" PRId64.
- Cast corresponding argument to (int64_t).

Fix uses of plain "%ld" within sprintf():
- Replace "%ld" with "%" PRId64.
- Cast corresponding argument to (int64_t).