all: assorted cleanups and updates

Changes include:
  - add a bunch of "noexcept", mostly to operators and forceinline
  - use "uchar"
  - use "charptr"
  - rename options_t to Options
  - add ptr_check_no_overlap()
  - rewrite p_exe.cpp, NFCI
  - clang-format help.cpp
  - spelling fixes

src/util/xspan_impl.h

@@ -52,11 +52,11 @@ void xspan_check_range(const void *p, const void *base, ptrdiff_t size_in_bytes)
 
 // help constructor to distinguish between number of elements and bytes
 struct XSpanCount {
-    explicit XSpanCount(size_t n) : count(n) {}
+    explicit XSpanCount(size_t n) noexcept : count(n) {}
     size_t count; // public
 };
 struct XSpanSizeInBytes {
-    explicit XSpanSizeInBytes(size_t bytes) : size_in_bytes(bytes) {}
+    explicit XSpanSizeInBytes(size_t bytes) noexcept : size_in_bytes(bytes) {}
     size_t size_in_bytes; // public
 };