[[ Cleanup ]] Change MCExecEnumTypeElementInfo.value to type intenum_t

The value field of the MCExecEnumTypeElementInfo struct represents a
C++ enum value.  It should therefore be of type intenum_t, not
uindex_t.
[[ Cleanup ]] MCButton::mfocus(): Remove set-but-unused variable.
[[ Cleanup ]] MCCard: Add some disambiguating parentheses/braces.
[[ Cleanup ]] MCCardList: Add some disambiguating parentheses.
[[ Cleanup ]] MCCdata::load(): Use a reinterpret_cast from integer to void *.

This will work correctly even when sizeof(int) != sizeof(void *).
[[ Cleanup ]] MCChunk: Add some disambiguating parentheses/braces.
[[ Cleanup ]] MCSort::parse(): Add some disambiguating parentheses.
[[ Cleanup ]] MCWait::parse(): Reformat if statements for greater clarity.

Makes the code easier to read *and* silences some warnings!
[[ Cleanup ]] MCMarking::parse(): Add some disambiguating parentheses.
[[ Cleanup ]] MCAutoArray::Size() returns a uindex_t.
[[ Cleanup ]] Remove unused function replace_char_with_char().
[[ Cleanup ]] Remove unused function replace_char_with_char_caseless().
[[ Cleanup ]] Remove unused function replace_general().
[[ Cleanup ]] MCIdeScriptReplace: Correct constructor initialisation ordering.
[[ Cleanup ]] MCIdeScriptFlush::parse(): Remove set-but-unused variable.
[[ Cleanup ]] MCCacheHandle::GetFilePointer(): false is non-pointer type.
[[ Cleanup ]] MCMultiPartParseHeaderParams(): r_param_count is of non-pointer type.
[[ Cleanup ]] get_variable_ex_utf8(): false is non-pointer type.
[[ Cleanup ]] MCExternalVariable: Avoid double-dereferencing pointers.

Previously, MCExternalVariable's AppendInteger() and AppendCardinal()
methods each cast their 32-bit integer argument to a pointer and
dereferenced it, which was clearly the Wrong Thing on any non-32-bit
platform.

When I looked into this further, I found that both call sites for
these methods passed in values, not pointers, so the previous
behaviour was Obviously Wrong.

If this change causes regressions then we'll need to fix the API to be
64-bit safe and a lot less abusive.
[[ Cleanup ]] MCExternalVariableAppend(): Initialise 16-bit string correctly.

Prevent appending UTF16 strings from crashing the program due to an
incorrectly-initialised variable.