[WIP] 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`
- [ ] `ASTNegate`
- [ ] `ASTPick`
- [ ] `ASTPluck`
- [ ] `ASTReduce`
- [ ] `ASTReject`
- [ ] `ASTShuffle`
- [ ] `ASTSize`
- [ ] `ASTTail`
- [ ] `ASTUnion`
- [ ] `ASTWithout`
- [ ] Remove `ASTConstants.h`

(The naming scheme isn't final yet.)

Fixes #20