Change license to Apache 2 with LLVM exceptions

[mlund]

This changes the license of mega65-libc from LGPL to Apache version 2 with LLVM exceptions. This enables mega65-libc to be bundled with compilers such as llvm-mos-sdk and calypsi. For a detailed discussion and motivation of this change, see #49.

• Change LICENSE file to Apache 2 with LLVM exceptions
• Update Github PR check-list
• Remove license snippets from header and source files
• Approval from past contributors, in particular @gardners (creator) and @steph72 (fcio) and @hernandp (conio)

If you have previously contributed to the library, please place a comment stating if you (dis)agree with this change.

Closes #49.

[gardners]

I grant the license change on my contributions.

[lydon42]

@hernandp and @steph72: could you please also approve the license change for your contributions?

[steph72]

I grant the license change on my contributions.

[lydon42]

I grant the license change on my contributions (just for completeness -- I might be the one merging this anyway).

[ki-bo]

Just noticed there are commits from me, as well, so also just for completeness:
I grant the license change on my contributions.

[mlund]

Let's also ask @gurcei and @zeldin.

[zeldin]

I grant the license change on my contributions (as meager as they currently are 😆).

[hernandp]

I agree with the license change.