Don't allow macros with names starting with mnemonics except when -w is used

[ZornsLemma]

As discussed in https://github.com/stardot/beebasm/issues/80, unless -w is used we should disallow macro definitions which define a macro starting with a mnemonic, as attempts to invoke it are likely to be misinterpreted as a use of the relevant mnemonic. (This assumes -w has been extended to always require a space after the mnemonic; this is probably going to be implemented soon under https://github.com/stardot/beebasm/pull/81. Without this change other problems probably occur with such macro names.)

[mungre]

#81 is complete, -w always requires a space after a mnemonic.