Apply static inline to all methods

- [x] `ASTAll`
- [x] `ASTAny`
- [x] `ASTDefaults`
- [x] `ASTDifference`
- [x] `ASTEach`
- [x] `ASTEmpty`
- [x] `ASTExtend`
- [x] `ASTFilter`
- [x] `ASTFind`
- [x] `ASTFlatten`
- [x] `ASTGroupBy`
- [x] `ASTHead`
- [x] `ASTIndexBy`
- [x] `ASTIndexOf`
- [x] `ASTIntersection`
- [x] `ASTMap`
- [x] `ASTMin` & `ASTMax`
- [x] `ASTNegate`
- [x] `ASTPick`
- [x] `ASTPluck`
- [x] `ASTReduce`
- [x] `ASTReject`
- [x] `ASTShuffle`
- [x] `ASTSize`
- [x] `ASTTail`
- [x] `ASTUnion`
- [x] `ASTWithout`
- [x] ~~Remove ASTConstants.h~~ I ended up introducing a new `ASTERISM_OVERLOADABLE` in `AsterismDefines.h`

Refactoring `ASTNegate` also obviated the need for `metamacros.h`.

Fixes #20